Generation V
Defiant
When its stats are lowered its Attack increases.
Detailed Effect
When any of this Pokémon's stats are lowered, its Attack rises by two stages. If multiple stats are lowered at once, this ability takes effect with each stat lowered.
Pokémon With Defiant
17 canonical Pokémon forms can have this Ability.

Pawniard
Bisharp
Kingambit
Ogerpon
Galarian Zapdos 
Mega Falinks
Mankey
Primeape
Farfetchd
Purugly
Braviary
Tornadus Incarnate
Thundurus Incarnate 
Passimian
Obstagoon
Falinks
Annihilape