Realism? Hmmmm... Outlaw such monsters as Flumphs, Giant Space Hamsters, Owlphants, Stench Kow (unless you like skunky cows), hippomen, bowlers, Biclops, senmurv (unless the PC's are color blind), Flail Snail, wolf in sheep's clothing (although I have a soft spot for anything with bunnies)...