You need to breed a female Mightyena/Poochyena with a male Pokemon from the Field Egg Group that knows Fire Fang.
The following compatible fathers learn Fire Fang via levelling up:
Arbok
Growlithe
Arcanine
Flareon
Snubbull
Granbull
Houndour
Houndoom
Donphan
Exploud
Manectric
Hippowdon
Stoutland
Darumaka
Darmanitan
Litleo
Pyroar
You can also use a Snergle that knows Fire Fang through Sketch.
Alternatively you can use the DexNav to find a wild Poochyena/Mightyena with Fire Fang (this method is more luck-based)