Generation V
Big Pecks
Protects the Pokémon from Defense-lowering attacks.
Detailed Effect
This Pokémon's Defense cannot be lowered by other Pokémon. This Pokémon can still be passed negative Defense modifiers through Heart Swap or Guard Swap.
Pokémon With Big Pecks
15 canonical Pokémon forms can have this Ability.

Pidove
Tranquill
Unfezant
Ducklett 
Swanna
Vullaby
Mandibuzz
Fletchling
Bombirdier
Pidgey
Pidgeotto
Pidgeot
Chatot
Rookidee
Corvisquire