Anger Shell
When the Pokémon's HP drops below half, Anger Shell lowers its Defense and Special Defense but its Attack, Special Attack and Speed are raised.
Full effect
When an attack causes its HP to drop to half or less, the Pokémon gets angry. This lowers its Defense and Sp. Def stats but boosts its Attack, Sp. Atk, and Speed stats.
In game description
When an attack causes its HP to drop to half or less, the Pokémon gets angry. This lowers its Defense and Sp. Def stats but boosts its Attack, Sp. Atk, and Speed stats.
Pokemon with Anger Shell
Using Anger Shell in Cobblemon
Abilities are passive and always active, so Anger Shell 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.