Semi-Pseudo is a term used to describe a Pokémon who is often mistaken for a Pseudo-Legendary, but fail to meet the criteria to be a Pseudo Legendary. The most notable example of this lies in the fact that nearly all the Semi-Pseudo have a base stat total that does not equal 600, so they cannot be considered Pseudo Legendary by definition. Pseudo Legendary Pokémon are usually uncommon/rare.
Hoenn is the only region, as of yet, to contain 2 Pseudo-Legendary Pokémon instead of just 1.
Coincidentally, they are usually found in hard-to-find places. For example: Bagon (Salamence's basic stage) can only be found at the back of Meteor Falls Cave with the use of Surf and Waterfall, meaning that you'd need eight Gym Badges to even think about going there and Metagross can only be obtained after the player defeats the Elite Four and Steven, the player must then go to Steven's house, the player then receives a Beldum.
Flygon, Aggron and Slaking, all Semi-Pseudo Legendary Pokémon, are also found in Hoenn.
Lucario and Zoroark are mistaken as Pseudo-Legendary Pokémon because of the way in which they are to be obtained.
A Lucario can also be obtained in Pokémon X and Y at the Tower of Mastery after you beat the Shalour City Gym leader, Korinna and have obtained the Key Stone. You'll be instantly challenged to a Mega Lucario VS Mega Lucario battle against Korrina. They are also rarely found on the Poni Coast in Pokémon Sun and Moon as Riolu or by SOS battle with a Riolu.
Zoroark was once only able to be caught as an event. Because of this, it is mistaken as a Legendary. Zoroark could be obtained when either the event Raikou, Entei or Suicune was taken to Lostlorn Forest outside Nimbasa City in the first Black & White. It was also possible to obtain Zoroark's pre-evolution, Zorua, but Zorua needed an event Celebi to be captured in Black & White. However, in Black & White 2 the player is given a Zorua by the sage in Driftveil City provided they have room in their party for it to fit, it seems that it once belonged to N. In Generation VI player characters in the Pokémon Village or on Route 20 have a low chance of running into a wild Zoroark.
Tyranitar and Metagross are the only Pseudo-Legendary Pokémon that cannot learn Dragon-type moves by leveling up.
However, Metagross is the only Pseudo-Legendary Pokémon that cannot learn a Dragon-type move at all.
All Pseudo-Legendary Pokémon have been used in a battle with a champion at one time or another:
Lance has a Dragonite in Gold/Silver. Ryuki also has a Dragonite when he challenges the player in Sun and Moon.
Blue (Blue/Green) has a Tyranitar in the rematch in FireRed/LeafGreen.
Steven and Molayne (One of the challengers in Sun and Moon) have a Metagross.
Lance has a Salamence in the Pokémon Black and White 2 world tournament.
Cynthia and Ryuki (One of the challengers in Sun and Moon) have a Garchomp.
Ryuki (One of the challengers in Sun and Moon) has a Kommo-o.
Leon uses a Dragapult during the final champion battle.
Lance and Ryuki are the only Champions/Champion Challengers that use the most Pseudo-Legendaries on their teams, with 3 each.
Every Pseudo-Legendary Pokémon, at level 100, has a total experience of 1,250,000.
Goodra is the only Pseudo-Legendary Pokémon with a single type.
Out of the eight Pseudo-Legendary Pokémon, four of them can mega evolve, while two out of the twelve Semi-Pseudo Legendary Pokémon can.
Greninja is the only starter Pokémon to be a Semi-Pseudo Legendary Pokémon.
Slaking has the highest base stat total of all Pseudo Legendary and Semi-Pseudo Legendary Pokémon with a base stat total of 670.
Metagross is the only non-reptilian Pseudo-Legendary.
Most Pseudo-Legendaries and Semi-Pseudo Legendaries are based on giant Japanese monsters, also known as Kaiju's. Some examples are Tyranitar which is based on Godzilla and Volcarona which is based on Mothra.
All Pseudo-Legendary Pokémon have a 4x weakness with the exception of Metagross, Goodra, & Dragapult.