kobolds, rabid rats, baby green dragons, vampire bats (poisons you, no beneficial effect except nutrition; okay for poison-resistant characters like orcs, barbarians or if you have acquired poison resistance)
bats and stalkers (stuns you for a while; see above for benefits to eating stalkers)
mimics (you can't move for a while)
cockatrice, chickatrice, Medusa (you die of stoning; okay if you're petrification resistant)
Death, Famine, Pestilence (you die)
green slimes (slimes you)
your own race (cannibalism gives you the aggravate monster intrinsic unless you're a caveman or orc)