They are among the few mammals that lay eggs. Well, one of TWO main mammals in the WORLD that lay eggs. So they're pretty special. I wonder why the creators at Sega decided an Echidna would be a good idea--not like they're bad or anything, but really? WTF? The echidnas look nothing like IRL echidnas anyway.
So here's Tikal with an egg (relative to her size, I guess). Sonic's freaking out and Knuckles is just like, "Dude, STFU." xD
Baby echidnas are called Puggles. 8D