The platypus is one of the most unique creatures alive today. All mammals can be divided into three types, including placentals (most modern mammals), marsupials (many mammals from Australia, like kangaroos and koalas) and monotremes.
There are only two living species currently classified as monotremes, which are mammals which lay eggs. One of these is the platypus.
Not only does the platypus lay eggs, it is aquatic, furred, has a duck-like bill, and can produce venom. When mapping the platypus genome, scientists found that it has a mixture of reptilian, bird-like and mammal traits and probably split from the general mammal evolutionary path about 165 million years ago.
More about the platypus:
So, what is the other mammal that lays eggs?