Mule
From Equestripedia, the Archives of Equestria!
Mules are a species of equine known to inhabit Equestria. The name Mule is occasionally used as an insult.
Biology
Mules bear a strong resemblance to Donkeys, and indeed little serves to differentiate the species physically.
It is possible that Mules are the offspring of pairings between a Donkey and a Pony, based on how human world Mules come into existence.