In case you were wondering, both asinine and jackass are indeed donkey-based terms of insult. Asinine is the older term, dating to the 15th century and has an ancestor in the Latin word asinus meaning ass (which is as old as English itself). Asinine suggests utter and contemptible failure to use normal rationality or perception. Jackass comes from the word ass plus jack, which can mean "a familiar term of address for a social inferior." Jackass refers to a male donkey or a stupid person or fool. This noun entered English early in the 18th century.

If you're interested in being less obvious with your donkey-based insults, you might try onocentaur. That term names a mythological creature with the head, arms, and upper torso of a human being and the body and legs of an ass.