Axolotls are also cool because they're not really a species in their own right --- they're actually just the juvenile form of a salamander. But they can breed while in their juvenile form! It's as if tadpoles could lay eggs without having to turn into frogs first.

In fact, axolotls only metamorphose into salamanders if triggered by the external environment. If you don't stimulate them in the right way, they stay axolotls. For a long time they were thought of as two different types of creature completely.

There's a story of one of the very early researchers looking at his vivarium one day and discovering that it was now full of salamanders rather than the axolotls he was expecting. I would love to have heard the conversation...