This is the gastric-brooding frog, also known as the Platypus frog. There were two species of these things, and in both cases the female frog would swallow the fertilized eggs. The tadpole would then develop into a frog while in the female’s stomach.

These little weirdos went extinct sometime in the 1980s, although for once it wasn’t (entirely) humanity’s fault – we can lay the blame with chytrid, a nasty little fungus that has an affinity for messing frogs right the hell up.