Pokémon with this ability cannot have their Defense lowered by other Pokémon.
Pokémon with Big Pecks
As a natural ability
|If any of the Pokémon were introduced before the generation that Big Pecks was introduced in, then they will not have it as a natural ability until then.|
|Hidden abilities were not introduced until Generation V, so if any of the Pokémon were introduced before that, then they will not have Big Pecks as a hidden ability until then.|