Green’s Dictionary of Slang

(US) a mule, a donkey; thus long-eared adj.

1845		T.J. Green Journal of the Texian Expedition 249: The burra’s milk [...] to which add, after boiling it long enough to take off the ‘long-eared’ smell, two quarts of ass’s milk.
1889		Century Dict. 3511: Long-ears... A humorous name for a donkey.
1890–1904		Farmer & Henley Sl. and Its Analogues.
1944		R.F. Adams Western Words (1968) 184/1: Long-eared chuck wagon – A humorous name for the mules which pack the provisions when these animals have to be used instead of wagons in rough country.
1950		WELS n.p.: (Joking names for mules) 3 Infs, Longears; 1 Inf, Long-eared jack; 1 Inf, Long-eared s.b. – [heard in the] army; used occasionally.
1962		(con. 1800s) L.D. Wyld Low Bridge 23: Mules, in Erie parlance, said one informant, were ‘long-eared robins.’.