Even though it is PU, which is the tier for the mostly least used Pokemon, it can still pack a punch and is very powerful. If you're using it in an OU team, then don't use it because Azumarill will destroy it with Aqua Jet. But if you're using it in a lower tier, then I'd highly recommend it. Here is a good set for this Fire Horse:
Rapidash @ Life Orb
Trait: Flash Fire
Adamant Nature
EVs: 252 Atk, 252 Spe, 4 SpD
Moves:
Flare Blitz
Poison Jab
Quick Attack
Wild Charge
Hope I helped!