Mount for Quixote's squire

ASS

Think of the humble beast of burden often associated with stubbornness, which Sancho Panza famously rode alongside his idealistic master. It’s not a horse but shares the same family as a mule.

This clue refers to Sancho Panza, the squire of Don Quixote in Miguel de Cervantes' famous novel. Sancho’s loyal but stubborn mount is a donkey, which is colloquially called an "ass." The answer plays on the double meaning of "ass" as both an animal and a slang term, though here it strictly means the donkey.