Chihuahuas please help???!!!~~~?

ok so we were driving through town when we saw a small chihuahua running through the traffic so we followed it and eventually i caught it shes either a deer nosed chihuahua or a chihuahua x foxy anyways she is quite thin does anyone know of any fattening foods for small dogs???oh and she is not microchipped and no one seems to be looking for her at the pound or the RSPCA so fattening foods???????