Regenerator

Heals for 1/3 max HP upon switching out.

Full 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.

In game description

The Pokémon has a little of its HP restored when withdrawn from battle.

Pokemon with Regenerator

Using Regenerator in Cobblemon

Abilities are passive and always active, so Regenerator costs you nothing to run. If it is marked hidden on a species above, expect it to be harder to obtain: servers differ in whether hidden abilities come from breeding, an Ability Patch, or events.

See the Pokedex for which abilities each species can have, and IVs, EVs and breeding for how to actually pass one down.