Extreme Monotremes

5 species of egg-laying mammals are in existence today; 4 of these are species of echidna, or "spiny anteaters", 

while the other is the platypus, 

whose true scientific name is "Ornithorhynchus", meaning "duck-billed".

These animals are also unique to Mammalia in several other ways. For example,  while they do secrete concentrated milk from mammary glands, they do not possess nipples or teats, and instead the milk comes directly from the milk-line region of their skin. It is also notable that the name monotreme means "one opening" which refers to their common urogenital pore called a cloaca, a feature which suggests these species are the most primitive of the mammals around today. The males of these species also posses a poison spur on their hind leg (poison production is a very rare trait for mammals). One sting from a platypus spur can actually kill small pets!