11 species have Defiant as a hidden ability, 4 as a normal one

Effect, in PokéAPI's words

“Raises Attack two stages upon having any stat lowered.”

As a hidden ability

Species	Slot	Note
Mankey	Hidden	Since Generation V
Primeape	Hidden	Since Generation V
Farfetch’d	Hidden	Since Generation V
Purugly	Hidden	Since Generation V
Braviary	Hidden
Tornadus	Hidden
Thundurus	Hidden
Passimian	Hidden
Obstagoon	Hidden
Falinks	Hidden
Annihilape	Hidden

As a normal ability

Species	Slot	Note
Pawniard	Normal
Bisharp	Normal
Kingambit	Normal
Ogerpon	Normal

Held in earlier generations

Species	Slot	Note
Piplup	Former	Through Generation VIII

Elsewhere in PokéAPI

2 more holders without a page here

Alternate forms and other species that PokéAPI lists with Defiant.

Zapdos Galar, Falinks Mega