Defeatist
Halves Attack and Special Attack at 50% max HP or less.
Full effect
This Pokémon's Attack and Special Attack are halved when it has half its HP or less.
In game description
Halves the Pokémon’s Attack and Sp. Atk stats when its HP becomes half or less.
Pokemon with Defeatist
Using Defeatist in Cobblemon
Abilities are passive and always active, so Defeatist 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.