Platypus is among the rare species of animals known as egg-laying mammals otherwise known as monotremes.
Mammals are mainly categorized into three groups:
(i) Monotremes
(ii) Placentals e.g humans
(iii) Marsupials e.g kangaroo
Monotremes like every other mammal are hairy and all, but have different dentition and reproductive tract structure.
Platypus' mouth is shaped into what is called a bill; it can be regarded as their sixth sense as it contains thousands of cells that can detect prey in water by electrolocation. They lay eggs but nurse their young ones with milk from the mammary glands. Also, like the the name "monotreme" implies, they have only one opening (cloaca) on their body for their urinary, digestive, and reproductive systems.
The only animals that are monotremes are the Platypus and four other species of Echnida.