There are various possibilities here, including:
rainbow snakehead
African butterfly fish
some sort of puffer
one or two medium-sized Central American cichlids (nothing as big as an oscar though). The guys at New World Cichlids forum will help
unusual catfish- something like banjos or whiptails, or even unusual small plecs
African cichlids, like humpheads, they look quite unusual
halfbeaks
I know that most of these don't class as oddballs- but then pacus don't either so presumably your Dad does not insist on this