Regenerator

A Pokémon with Regenerator restores 1⁄3 of its maximum HP when switching out.

Game descriptions

Black, White, Black 2, White 2:Restores a little HP when withdrawn from battle.
X, Y, O.Ruby, A.Sapphire:Restores a little HP when withdrawn from battle.
Sun, Moon, Ultra Sun, Ultra Moon:Restores a little HP when withdrawn from battle.
Sword, Shield, B.Diamond, S.Pearl:Restores a little HP when withdrawn from battle.
Scarlet, Violet:The Pokémon has a little of its HP restored when withdrawn from battle.