Monotremes

The common name for monotreme is "single hole". This class includes platypi and echidnas.

Some of the defining features of monotreme include laying eggs instead of live birth, lacking a corpus callosum (which connects the two hemispheres of the brain), and having only one hole for reproduction, urination, and defecation. Monotremes are the only mammals that lay eggs, but the egg remains inside the female for some time in order to be nourished. When the baby platypus hatches, it is fed by milk from it's mother.

Here is a video of an echidna walking:
http://youtu.be/980mzboCctg