Harvest
Has a 50% chance of restoring a used Berry after each turn if the Pokémon has held no items in the meantime.
Full effect
After each turn, if the last item this Pokémon consumed was a Berry and it is not currently holding an item, it has a 50% chance of regaining that Berry, or a 100% chance during strong sunlight.
In game description
May create another Berry after one is used.
Pokemon with Harvest
Using Harvest in Cobblemon
Abilities are passive and always active, so Harvest 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.