Swift Swim

Swift Swim (すいすい Swift Swim) is an ability that doubles the user's Speed stat when it is raining.

Game descriptions

Generation	Description
III	Raises SPEED in rain.
IV	Boosts the Pokémon’s Speed in rain.
V	Boosts the Pokémon’s Speed in rain.
VI	Boosts the Pokémon's Speed stat in rain.
VII	Boosts the Pokémon's Speed stat in rain.

Pokédex	Pokémon	Sprite	type
#116	Horsea
#118	Goldeen
#119	Seaking
#129	Magikarp
#138	Omanyte
#139	Omastar
#140	Kabuto
#141	Kabutops
#211	Qwilfish
#226	Mantine
#230	Kingdra
#270	Lotad
#271	Lombre
#272	Ludicolo
#283	Surskit
#349	Feebas
#367	Huntail
#368	Gorebyss
#369	Relicanth
#370	Luvdisc
#418	Buizel
#419	Floatzel
#456	Finneon
#457	Lumineon
#458	Mantyke
#535	Tympole
#536	Palpitoad
#537	Seismitoad

Pokédex	Pokémon	Sprite	type
#054	Psyduck
#055	Golduck
#060	Poliwag
#061	Poliwhirl
#062	Poliwrath
#347	Anorith
#348	Armaldo
#564	Tirtouga
#565	Carracosta
#614	Beartic

Pokédex	Pokémon	Sprite	type
#260	Swampert

The three non-Water-type Pokémon to have Swift Swim as their Ability have it as a Hidden Ability.
In Pokémon Mystery Dungeon, when a Pokémon with this ability uses a movement and it's raining, the movement is repeated again without spending PP on repetition.