Both are Bug-type Pokémon with a 3-stage evolution, starting off as a worm-like Pokémon, evolving into a cocoon-like Pokémon, and then finally evolving into a winged insect. Both of them evolve once at Level 7 and then fully evolve at Level 10. Both carry the same base stat total through each stage of their evolutionary lines. The Caterpie line is more common in Blue while the Weedle line is more common in Red. This is only true for Metapod and Kakuna in the remakes. Each of their final forms also gain a different form in different generations. Butterfree can Gigantamax in Generation VIII while Beedrill can Mega Evolve in Generation VI.
While they are almost the same, they are classified as two separate Pokémon. Both are pure Poison types that evolve at Level 16, and then fully evolve again with a Moon Stone, in which they become Poison/Ground-type Pokémon. Nidoran♂ is more common in Red and FireRed while Nidoran♀ is more common in Blue and LeafGreen.
Clefairy and Jigglypuff
Both are Fire types that evolve using the Fire Stone. Growlithe is exclusive to Red, Gold, FireRed and HeartGold, while Vulpix is exclusive to Blue, Silver, LeafGreen and SoulSilver. Both require a Generation III game (FireRed for Growlithe, LeafGreen for Vulpix) to be inserted into the DS in order to be encountered in Sinnoh.
Oddish and Bellsprout
Both are three-stage evolution Pokémon that reach their final form after having their middle-evolution being traded. Both of their final evolutions also gained a Gigantamax form in Generation VIII that are the ace Pokémon of their Version Exclusive Gym Leaders (Bea uses Gigantamax Machamp while Allister uses Gigantamax Gengar).
They are both Poison-types in the Amorphous egg group. Grimer is more common in Blue, Sapphire, LeafGreen and Alpha Sapphire while Koffing is more common in Red, Ruby, FireRed and Omega Ruby. Both are also based on different kinds of pollution (Grimer is based on land pollution while Koffing is based on air pollution). Both of their final forms also gained Regional form that give them a second type. Muk received one in Generation VII while gaining the Dark type while Weezing received one in Generation VIII while gaining the Fairy-type.
Hitmonlee and Hitmonchan
Both are pure Fighting-type Pokémon that can be rewarded at the Fighting Dojo in Red & Blue. Hitmonlee is based on the fighting style of kicking and using your feet while Hitmonchan is based on the fighting style of punching and using your fists. Both of their names are also based on famous martial artists with Hitmonlee is based on Bruce Lee while Hitmonchan is based on Jackie Chan. Both also gained the pre-evolution Tyrogue in Generation II.
Both are pure Water types in the Water 3 egg group and evolve using the Water Stone. Both of their evolutions are dual-typed (Cloyster gains the Ice type and Starmie gains the Psychic type). Cloyster is meant to be a slow defensive Pokémon and Starmie is meant to be a fast attacking Pokémon. Shellder is exclusive to FireRed and Y while Staryu is exclusive to LeafGreen and X.
They are next to each other in the Johto Pokedex and gained an evolution in Generation IV. They both evolve once they learn a specific Rock-type move (Ancient Power for Tangela and Rollout for Lickitung).
Electabuzz and Magmar
Both in the Human-Like egg group. Both received baby forms in Generation II and evolutions in Generation IV (by trade while holding a specific item). Electabuzz is exclusive to Red, FireRed and White 2 while Magmar is exclusive to Blue, LeafGreen and Black 2.
They are the first fossil Pokémon introduced. Both are Rock/Water types in the Water 3 egg group that evolve at level 40. In Generation I, FireRed and LeafGreen, one cannot obtain both without trading. Omanyte is exclusive to HeartGold and Alpha Sapphire while Kabuto is exclusive to SoulSilver and Omega Ruby. Both are exclusive to Pokémon X as well. Both also have Swift Swim and a critical hit-blocking ability (Shell Armor for Omanyte, Battle Armor for Kabuto) as their abilities. Both carry the same base stat total through each stage of their evolutionary lines.
Both are Bug types that evolve once by level-up. Both are version exclusive (with Ledyba in Silver and SoulSilver and Spinarak in Gold and HeartGold). Spinarak can only be encountered at night while Ledyba can only be encountered in the morning. In Sinnoh, only their evolved forms can be encountered.
Both evolve from Eevee during different times of day (Espeon during the day, Umbreon during the night). Both require high friendship in order to evolve from Eevee. Espeon learns Morning Sun while Umbreon learns Moonlight, both healing moves with different effectiveness during different times of day (in Generation II only).
Murkrow and Misdreavus
Both are Pokémon associated with darkness or scary things (Misdreavus is a ghost while Murkrow is a crow). They originally didn't evolve, until both received evolutions in Generation IV (by use of a Dusk Stone). Murkrow is exclusive to FireRed, Diamond and Black while Misdreavus is exclusive to LeafGreen, Pearl and White.
Both are Pokémon known for the powers of their claws. They originally didn't evolve, but both gained evolutions in Generation IV, evolving by leveling up at night while holding a specific item. Both carry the same base stat total through each stage of their evolutionary lines. However, it should be noted that Gligar was seen as the counterpart to Delibird in the original Gold and Silver games, sharing the type and being found in very close locations, the Ice Path for Delibird and Route 45 (South of Blackthorn City) for Gligar.
Both are Bug types with a base stat total of 500, and both gained Mega Evolutions in Generation VI. In Hoenn, both are found in different areas of the Safari Zone that require different types of bike to access. Pinsir is exclusive to Pokémon X while Heracross is exclusive to Pokémon Y.
Farfetch'd and Corsola
Both are dual-type Pokémon that do not evolve. Both are also hunted and go endangered due to hunting (Farfetch'd are hunted by humans while Corsola are hunted by Mareanie). Both also got a boost to their stats in Generation VII. In Generation VIII, they both gained a Galarian Form and a brand-new evolution.
Teddiursa is exclusive to SoulSilver while Phanpy is exclusive to HeartGold. They are also version exclusive in Gold and Silver but whichever game they are exclusive to depends on the localization of the game (e.g. Teddiursa is exclusive to Japanese and Korean localizations of Silver while Phanpy is exclusive to all other localizations of Silver). Both are in the Field egg group and have Pickup as their Ability. Their evolutions have different abilities.
Both are based on bovine creatures and can only be one gender. Both also have a base stat total of 490. They are one of the less commonly acknowledged counterparts but often appear next to each other in some regional Pokédexes.
These legendary version mascots are counterparts and both are leaders of Legendary Trios (Lugia is the master of Articuno, Zapdos, Moltres and Ho-Oh is the master of Raikou, Entei and Suicune). However, neither are exclusive to the versions they are mascots of (although they can be encountered at a lower level in those versions).
Both are Dark types based on canine animals. Poochyena is exclusive to Diamond and White while Houndour is exclusive to Pearl and Black. Both are exclusive to Pokémon X as well.
Both are early-encounter Pokémon that attack Professor Birch (Poochyena in Ruby & Sapphire and Zigzagoon in Emerald). Both also evolve once.
Both are Grass types, version exclusives and evolve under the same conditions (to get to the second stage, they need to level up to level 14 and to their final stage, they need to have a type of stone used on them). Both require the Generation III game they're exclusive to, to be inserted into the DS in order to be encountered in Sinnoh. Both carry the same base stat total through each stage of their evolutionary lines. Both have an ability that doubles their speed during a weather condition.
Sableye is exclusive to Sapphire, SoulSilver and Alpha Sapphire while Mawile is exclusive to Ruby, HeartGold and Omega Ruby. Both require the Generation III game they're exclusive to, to be inserted into the DS in order to be encountered in Sinnoh. Both have a base stat total of 380. Both are dual-typed (as of Generation VI) and both also received Mega Evolutions in Generation VI (although Mawile's was released before Sableye's).
Despite having no relation in terms of evolution, they look almost identical and their abilities used to only work when used together. Minun are more common in Ruby and Plusle are more common in Sapphire. In Omega Ruby, Plusle can only be encountered through hordes while this is the case for Minun in Alpha Sapphire. In Black and White they are fully version-exclusive.
Both are Bug-type Firefly Pokémon, have a base stat total of 400 and can only be one gender. Unlike with Tauros and Miltank, one can hatch Volbeat or Illumise from any egg that Illumise breeds with a compatible Pokémon.
Both are dual-type Pokémon that evolve once and are used by villainous teams in the generation they were introduced in (Team Magma uses Camerupt because of its Fire type and Team Aqua uses Sharpedo because of its Water type).
The first known rivalry between two different species of Pokémon. Hence, both are version exclusive in Generation III, and both require the Generation III game they're exclusive to, to be inserted into the DS in order to be encountered in Sinnoh. However, both have a base stat total of 458, and both belong to the Field egg group and the Wild Plains friend area in Pokémon Mystery Dungeon: Red Rescue Team and Blue Rescue Team.
They are both genderless, both are Rock/Psychic types, both version exclusive and represent the sun and moon, which are opposites. Both have a base stat total of 440. Both require the Generation III game they're exclusive to, to be inserted into the DS in order to be encountered in Sinnoh.
They are fossil Pokémon based on marine organisms. Thus, both are partly Rock type and belong to the Water 3 egg group. Both evolve at level 40, and both carry the same base stat total through each stage of their evolutionary lines. While not version-exclusive in those games, one cannot obtain both in Ruby and Sapphire without trading. Both are exclusive to Pokémon Y.
Both are weak Water-type Pokémon based on common fish. They have the same BST and share the same HP & Speed stats. Their Attack, Special Attack, Defense and Special Defense stats are also switched. Magikarp is considered common and Feebas is considered rare. Both can evolve once into dragon-like Water-type Pokémon that share the same BST, HP and Speed stats. They also have switched Attack, Special Attack, Defense and Special Defense. Both of them evolve once (Gyarados for Magikarp and Milotic for Feebas). Gyarados and Milotic also share the same BST and share the same stats swithces and similarities as their pre-evolved forms.
Both are Ghost types that evolve at level 37. Both have a base stat total of 295, with their evolutions' stats totalling up to 455. Duskull gained a further evolution in Generation IV, while Shuppet's evolved form received a Mega Evolution in Generation VI. Duskull is more common in Ruby and Omega Ruby while Shuppet is more common in Sapphire and Alpha Sapphire.
Both are part of a three-stage evolution that, when fully evolved, are Pseudo-Legendary Pokémon. Both are also introduced in the same generation.
Both are Dragon/Psychic-type Pokémon, gender-exclusive and version-exclusive. In Generations III and IV, one must roam in order to capture whichever is exclusive to their game. An event allows the other (that would otherwise be unavailable) to be encountered and caught.
Both are Fossil Pokémon (thus Rock type) that are version exclusive. Both are in the Monster egg group and evolve at level 30.
Both version exclusive (Stunky in Diamond and Glameow in Pearl) and both are used by Team Galactic's Jupiter and Mars respectively. They are also both a two-stage evolution and part of the Field egg group. Both Pokémon and their evolution are unavailable in Platinum.
Both are pre-evolutions of earlier generation Pokémon. Both evolve by level-up after learning Mimic (and they can also learn Copycat). Neither are strictly version-exclusive; while Mime Jr. is normally found in Diamond and Bonsly in Pearl, one can obtain Mime Jr. in Pearl or Bonsly in Diamond through the Trophy Garden event unlocked after obtaining the National Pokédex.
Version mascots for Diamond and Pearl control time and space respectively which are often used together.
Both are related to the moon, Darkrai representing the new moon and Cresselia representing the full moon. Darkrai causes bad dreams and Cresselia cures them.
They both represent the sea and they are parent and child that cannot evolve.
Both are Fighting type and look similar as if wearing a judo uniform with a black belt. Both have a base stat total of 465. While not version-exclusive in Generation V, in Black and Black 2 Throh can only be encountered in rustling grass. This is true for Sawk in White and White 2. Both became version-exclusive in Generation VI.
Both are Psychic types in a 3-stage evolution; the second stage is reached at level 32, and the final stage at level 41. Version exclusives.
These fossil Pokémon are based on early turtles and birds. They became version-exclusive starting from Omega Ruby and Alpha Sapphire.
Karrablast and Shelmet
Two Bug-type Pokémon that can only evolve when traded with each other. They also share the same HP and Special Attack stats.
A male and female counterpart pair who are based on American settlers and Native Americans. Bothtypes that evolve at level 54. They are also version exclusives.
These are rivals like Seviper and Zangoose but are NOT version exclusive. However, they both have a base stat total of 484, a base experience yield of 169 and a catch rate of 90.
Both are version-exclusive legendary Pokémon that represent Yin and Yang respectively.
Both are pure Fairy types that are version exclusives. They evolve once by trading while holding a signature held item. They also both have a veil-related Ability (Spritzee has Aroma Veil while Swirlix has Sweet Veil).
Both are Water-type Pokémon that have the same catch rate and evolve once. Both also have the same HP stat. Version Exclusives.
These fossil Pokémon represent aggression and passion. Both evolve at level 39 but at different times of day. They are also based on different groups of dinosaur (Tyrunt from theropods, Amaura from sauropods).
Both are legendary Pokémon and part of the Aura Trio, representing the letters X & Y. Xerneas is said to represent life and growing while Yveltal is said to represent death and destruction. Both also have different aura-related abilities named after the type they have (Xerneas has Fairy Aura while Yveltal has Dark Aura). Both also have opposite colors, with Xerneas being blue and Yveltal being red.
Both are early-game Normal-type Pokémon that appear during different times of day (Alolan Rattata by night, Yungoos by day). Both have the same BST and capture rate and evolve at Level 20. Their evolved forms are also showcased as Totem Pokémon in both games (Alolan Raticate in Sun & Ultra Sun and Gumshoos in Moon & Ultra Moon).
Both are unevolving Pokémon based on primates. Both of them have the same BST. Both also cancel each other's types out (Oranguru's Normal type is weak to Passimian's Fighting type while Passimian's Fighting type is weak to Oranguru's Psychic type). Version Exclusives.
Both start off as weak Water-type Pokémon that evolve once. Both are dual Water-type Pokémon that are some of the strongest non-legendary Pokémon of their generation. Both are also used by leaders of a villainous team (Lysandre uses Gyarados and Guzma uses Golisopod).
Turtonator and Drampa
Both are dual-type Dragon-type Pokémon. They do not evolve or have a pre-evolution and both of them are version exclusives. They also share the same catch-rate, BST, and moveset levels.
Legendary counterparts representing day and night. Both are dual-type Psychic types that both evolve from Cosmog & Cosmoem. They are Version Exclusives and only evolve into one another depending on which games they evolve in.
Both are Bug/Fighting-type Ultra Beasts that are Version Exclusives. Both are based on the fight/flight response (Buzzwole is bulky at fighting, a trait used with fight, and Pheromosa is quick at running, a trait used with flight).
Celesteela and Kartana
Both are dual Steel-type Ultra Beasts that are Version Exclusives. Both have the same BST as well. Celesteela is one of the heaviest Pokémon and is made to have high defense while Kartana is one of the lightest Pokémon and is made to have high attack.
Stakataka and Blacephalon
Both are dual-type Ultra Beasts that are Version Exlcusives. Both have the same BST as well. Stakataka is supposed to represent heavy things and weight while Blacephalon is supposed to represent light things and weightlessness.
Both are Partner Pokémon of their normal variants and are the Starter Pokémon of the Generation VII games Pokémon: Let's Go, Pikachu! and Let's Go, Eevee!. Both Partner Pikachu and Eevee are considered special because they follow you and cannot evolve in the game that they're the Starters of. Both Partner Pikachu and Eevee can be either gender, have higher base stats, learn Secret Techniques and have access to exclusive moves that normal Pikachu and Eevee do not. Both Partner Pikachu and Eevee are only exclusvie to their respective games that they are received in as the Starters of the game, Partner Pikachu can only be received in Pokémon: Let's Go, Pikachu!, while Partner Eevee can only be received in Pokémon: Let's Go, Eevee!.
Both are three-stage evolution Pokémon that are dual-Fairy types once they fully evolve. Both are related to emotions (Hatenna can sense emotions while Impidimp absorbs negative emotions). Both have gradually growing hair on their body to the point of it covering the entire Pokémon. Both are also gender-exclusives with the Hatenna-line being female and the Impidimp-line being male.
Stonjourner and Eiscue
Both are mono-type Version Exclusive Pokémon that don't evolve. Both have the same BST and catch rate. Both are also the second strongest Pokémon of the two version exclusive Gym Leaders that use them (Stonjourner for Gordie, who appears in Pokémon Sword and Eiscue for Melony, who appears in Pokémon Shield).
Both start off as mono-type Legendary Pokémon that are Version Exclusives. Upon being given an item, they gain the Steel type. Both of their Shiny variants match the color their box-art represents (Zacian being blue and Zamazenta being red).
Both of the legendary giants can be encountered in the Split-Decision Ruins; the opposite Pokémon cannot be encountered once the giant is chosen.
One of the two mono-type Horse Pokémon were once Calyrex's loyal steed. Using the Reins of Unity, Glastrier and Spectrier can fuse with Calyrex to become Ice Rider Calyrex and Shadow Rider Calyrex respectively. Once the steed is chosen, the opposite Pokémon cannot be encountered.