donkey

The origin is uncertain. Originally a slang term from the late eighteenth century. Perhaps from Middle English *donekie (“a miniature dun horse”), a double diminutive of Middle English don, dun, dunne (a name for a dun horse), equivalent to modern English dun (“brownish grey colour”) + -ock (diminutive suffix) + -ie (diminutive suffix), or similarly formed from the given name Duncan. Compare Middle English donning (“a dun horse”), English dunnock. Became more common than the original term ass due to the latter's homophony and partial merger with arse (compare similar development between coney and rabbit).

来源：wiktionary