6 species have Flash Fire as a hidden ability, 19 as a normal one

Effect, in PokéAPI's words

“Protects against Fire moves. Once one has been blocked, the Pokémon's own Fire moves inflict 1.5× damage until it leaves battle.”

As a hidden ability

Species	Slot	Note
Cyndaquil	Hidden	Since Generation V
Quilava	Hidden	Since Generation V
Typhlosion	Hidden	Since Generation V
Rolycoly	Hidden
Carkol	Hidden
Coalossal	Hidden

As a normal ability

Species	Slot	Note
Vulpix	Normal
Ninetales	Normal
Growlithe	Normal
Arcanine	Normal
Ponyta	Normal
Rapidash	Normal
Flareon	Normal
Houndour	Normal
Houndoom	Normal
Heatran	Normal
Litwick	Normal
Lampent	Normal
Chandelure	Normal
Heatmor	Normal
Sizzlipede	Normal
Centiskorch	Normal
Charcadet	Normal
Armarouge	Normal
Ceruledge	Normal

Held in earlier generations

Species	Slot	Note
Entei	Former	Through Generation VI

Elsewhere in PokéAPI

4 more holders without a page here

Alternate forms and other species that PokéAPI lists with Flash Fire.

Coalossal Gmax (hidden), Centiskorch Gmax, Growlithe Hisui, Arcanine Hisui