Your parent's male rabbit.
If your parents paid for it, its your bunny brother.
If you paid for it, its your bunny.
If your sibling paid for it, its your nephew-bunny.
A short, convienient way to say "The bunny my parents adopted for me, but its still technically their bunny because they paid for it. I call it my bunny sometimes but my parents pay for all of her stuff. So hes like my brother except for the fact that he is a bunny."
A wonderful compound word that is easy to understand even if you didn't look it up on urbandictionary.
Bunny + Brother = He is a bunny and he is your brother.