Donkey
late 18th century (originally pronounced to rhyme with monkey ): perhaps from dun1, or from the given name Duncan .
wiktionary
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). Compare Middle English donning(“a dun horse”), English dunnock.
etymonline
donkey (n.)
familiar term for an ass, 1785, also donky, donkie, originally slang or dialectal, of uncertain origin. Perhaps a diminutive from dun "dull gray-brown" (from Middle English donned, past participle of donnen "to lose color, fade, from Old English dunnian). Compare Dunning, name of a (dun) horse (mid-14c.), and see dun (adj.). The form perhaps was influenced by monkey.
Or perhaps it is from a familiar form of the proper name Duncan applied to an animal (compare dobbin). The older English word was ass (n.1). Applied to stupid, obstinate, or wrong-headed persons by 1840. In mechanics, used of small or supplementary apparatus from mid-19c. (donkey-engine, donkey-pump, etc.). Short form donk is by 1916.
