Generation V
Regenerator
Restores a little HP when withdrawn from battle.
Detailed Effect
This Pokémon regains 1/3 of its maximum HP when it is switched out for another Pokémon under any circumstances other than having fainted. This ability does not take effect when a battle ends.
Pokémon With Regenerator
28 canonical Pokémon forms can have this Ability.

Audino
Mienfoo 
Mienshao
Gossifleur
Eldegoss
Hydrapple
Tornadus Therian
Mega Dragalge
Slowpoke 
Slowbro
Tangela
Slowking
Corsola
Ho Oh
Tangrowth
Solosis
Duosion
Reuniclus
Foongus
Amoonguss
Alomomola
Mareanie
Toxapex
Klawf
Cyclizar
Galarian Slowpoke
Galarian Slowbro
Galarian Slowking