I put '15th' donkey is bad instead of 'fifteenth', could both be accepted?
Donkey and ass are interchangeable in English. Are they separate in Romanian?
No. We have intermediary words to name crosses between donkeys, mules, horses, but they are generally interchangeable unless you want to be specific.
OTOH I got this sentence as ”translate to English” from Ro, and I came here because I wanted to comment that a better translation would have been ”the fifteenth donkey is an ass”, hehe... kidding...