Green’s Dictionary of Slang

long-eared chum n.

(US) a mule.

1882		Nat. Police Gaz. (NY) 28 Jan. 3/4: They are especially proud of the dance which this long-eared friend of theirs has been taught to perform .
1960		A. Lomax Folk Songs of North America 429: That [...] critter, known to Americans variously as the barnyard yodeller, the hard-tail, the jug-head, the long-eared chum and the Missouri humming bird.