Freshwater Lobster (may attack other fish)
Kuhli Loach (not an oddball but they are quite different)
Upsidedown Catfish (Easy to keep and they swim upsidedown)
Chaca Chaca (Grows to 9 inches and will hide in sand until a fish swims to close to its mouth and then pounce. You could keep 2 in a 20 gallon long)
Albino Frog (May try to eat fish)