このロバめ!
Donkey in Japanese is "roba". I wrote it in katakana above. Roba has a difficult kanji (驢馬) which you could use, but a lot of people couldn't read it. (I actually didn't know such a kanji existed. I couldn't read/write it. It just pops up in the Microsoft kanji converter, which is how I found it.).
I don't know if it is a good/acceptable insult in Japanese. Seems OK, but you'd need a native to weigh in on something like this. Insults are super language-specific.