Short-Beaked Echidna

The Short-Beaked Echidna is one of the most bizarre mammals in existence. It, along with four other Echidna species and the Duck-Billed Platypus, are the world's only living Monotremes. Monotremes are considered to be very primitive mammals, in that they lay eggs in order to reproduce. All other mammals practice placental birth. There has been some arguement about where Monotremes should be placed taxonomically. Some consider them to be a separate, sister class to mammals, while others believe them to be reptiles (due to additional similarities in the digestive and excretory systems). Currently, they are placed as an Order within a separate subclass within Mammalia.

Image from Wildlife Queensland

The Short-Beaked Echidna (Tachyglossus aculeatus) is found throughout Australia and parts of New Guinea. There are five different subspecies that inhabit various ranges throughout. Within their range, the Short-Beaked Echidna  lives in a multitude of different habitats, ranging from mountain areas to deserts, and everything in between. They are even able to tolerate snow and cold weathers. They prefer to live in burrows, favoring fallen logs, caves, or even self dug burrows in the ground.

Short-Beaked Echidnas have clawed front feet that are exceptional for digging. These feet also help them to locate their favorite meals - ants and termites - so that they can slurp them up with their long tongues. Short-Beaked Echidnas have no teeth. Their bodies are covered with spines, interspersed with hair. These are used for defense. Short-Beaked Echidnas have compact, muscular bodies, and are able to roll up into a ball that is extremely difficult for predators to penetrate. They have wonderful senses of smell, but poor eyesight. Short-beaked Echidnas are warm blooded, and they do in fact produce milk for their young.

Short-Beaked Echidnas are typically very shy, solitary creatures, and will only come together to mate. After mating, the female will lay a single eggs, which is incubated in a pouch until hatching. The baby (unofficially called a puggle!) will remain in the pouch (similar to marsupials) until it's spines begin to grow. At that point it is raised in an external burrow. Short-Beaked Echidnas are the only Monotremes that are commonly found within captivity. They have also been bred in captivity.

Thanks to @iflylikeicarus for the suggestion!