A monotreme is a rare sort of mammal found in Australia, Tasmania and New Guinea. The reason they are so rare is that they lay eggs!
This group includes the Platypus and the Echidna (spiny anteater).
The platypus is a semi-aquatic creature with webbed feet, a bill (sort of like a duck), waterproof fur and a flat tail (like a beaver). They are between 1 and 2 feet long and between 3 or 4 pounds. Some of their bones are a lot like a reptile's and it lays eggs like a reptile. But it's still considered a mammal. It eats insect larvae, shrimp, frogs and fish eggs.
Echidna's have spines and VERY long, sticky tongues that they use to catch termites and ants. Echidna's are also between 1 and 2 feet long.