The Siamese Fighting Fish, beloved of children and vain adults all over the world. These guys are famous for their aggression - two males will, it is widely believed, fight to the death when housed together.
They're actually pretty interesting, colour and fin morphology aside. The males build a bubble-nest, and place fertilized eggs into the nest after mating with the female. The female just drops her eggs and retreats - males apparently are so aggressive they'll rip into a female shortly after mating. Juveniles resemble tadpoles - a common feature of fish, actually, since that's what baby guppies look like to me, as well.