From Popular Science, a story about one weird animal…
Really, almost everything about platypuses defies how we think about most mammals.
They do give milk to their babies, but unlike almost all other mammals they don’t have nipples. Instead, they essentially sweat out their milk from pores along their stomachs. The platypus has a bill kind of like a duck, but it’s really more of a hard snout. Their nostrils are on top of the snout, the mouth on the bottom, and oh yeah, they also sense their prey by detecting electrical fields. They literally close their eyes, ears, and nose when they dive underwater and go mainly on electroreception.
It also has some bonus bones in its shoulder not found in any other mammals, and rather than having its legs mounted beneath the body, its appendages spring out from the sides like a reptile. That means they also don’t swim like other mammals, who tend to use all four limbs. Platypuses let their back legs dangle—even though they, too, are webbed—and propel themselves entirely with the front feet, steering with the tail.