Ponyta are a pokemon once found in open plains and wild grasslands, but which declined in the wild (at least in Kanto) as those lands were vanquished in the march of civilization. They were eventually, after generations of decline, hunted to extinction as a means of denying nomadic raiders their mounts. They are, however, a commonly domesticated pokemon, much like Growlithe, Miltank, or Pikachu, and a feral population of Ponyta survives in the strange habitat of the pokemon mansion.
Traditionally herbivorous, it was once thought that they survived on various forms of plant life growing on the walls and windows, but this is insufficient to support the population within the mansion. Currently, they are believed to be a carnivorous breed which subsist on Rattata, which they trample to death with their diamond hooves and cook with their own flames.
Furthermore, these Ponyta, as their dispersal throughout the mansion demonstrates, are capable of charging through doors and climbing stairs, and they do not appear to possess the instinct of fear. Many have opined that it is lucky for all of us that the scientists of the pokemon mansion disappeared chasing Mew; had they stuck with Ponyta, the world would by now be faced with a cavalry in Team Rocket's hands which could run through any barricade and climb any fort. If the experimentation had been allowed to continue, the world would now be faced with a breed far more terrifying than even the barbarian Ponyta of old.