There's no limit to the number of bizarre adjectives you could use to describe a platypus. They have a snout like a duck but a hairy body like a mammal; they lay eggs and are one of the only venomous mammals in the world.
Even more bizarre, they have 10 sex chromosomes as opposed to the two (X and Y) that most other mammals have. It's no surprise that they are the sole living representative of the family Ornithorhynchidae.
Platypuses and echidas are the only egg-laying mammals on Earth, making them extremely valuable for studying mammalian evolution. Platypus-like fossils that date as far back [PDF] as at least 167 million years ago.