From Longman Dictionary of Contemporary Englishdonkeydon‧key /ˈdɒŋki $ ˈdɑːŋki/ ●○○ noun 1 
HBA[countable] a grey or brown animal like a horse, but smaller and with long ears2 → donkey’s years
HBA[countable] a grey or brown animal like a horse, but smaller and with long ears2 → donkey’s yearsExamples from the Corpus
donkey• The students dissected donkeys rather than horses and even Coleman used donkey material to demonstrate horse muscles.• Shelter is essential - especially for donkeys.• When he made his rounds on a light-gray donkey, people bowed their heads and dared not look at his face.• Police in Ballymoney, Co Antrim, yesterday confirmed the donkey was stolen.• The donkey decided to go to Bremen, a town where everyone played music.• The rugged terrain is inhabited by deer, antelopes and, locals say, wild donkeys.Origin donkey (1700-1800) Perhaps from dun (=gray-brown) + -key (as in monkey) 