Why is the plural of 'Donkey' 'Donkeys' and not 'Donkies'?

[EachPeachPearMum]

Erm... what is it for monkey [pg brain emoticon]?

Is it because the plural of key is keys?

I think the "e" makes the difference.... so you have keys, monkeys, donkeys. But you have mummies, puppies, etc. {hmm}

hmm- yes, thank you!
As I said... brain isn't functioning particularly well at present.