I pronounce Donkey to rhyme with Monkey; they both have short-U sound.
This is an endless source of amusement for my children, They pronounce it with an ON instead of my short-U. Of course, this means that they should also be calling that cute furry creature scampering through Queen Elizabeth's castle in Sea Hawk a mONkey.
Don't you agree? Munkey and Dunkey.... no one calls it a mONkey, now, do they?
There are pronunciation guides which purport to put this problem to rest. This one and this one seem to agree with my offspring. If only my parental units were still around to explain the realities of growing up on Long Island to my Chicago and California raised kids. They'd understand it in a heartbeat.... before reminding me how weird Daddooooo and G'ma talk.... that New York accent gets them every time.
Just a little something to mull over on Wednesday. It's 120 degrees outside, according to the thermostat in my rental car; my brain is stuck at the monkey/donkey dilemma. It's too hot to think more deeply that that.