Did you know that the word "donkey" used to rhyme with "monkey"? If not, you do now, and I hope you feel better for it.

"Donkey" is quite a new word. From Online Etymology:

1785, slang, perhaps from dun "dull grey-brown," the form perhaps infl. by monkey. Or possibly from a familiar form of Duncan (cf. dobbin). The older Eng. word was ass.

For some monkeying around with "asses" see here. For assing around with monkeys see here.