When I was growing up, I was taught that only the mammal that lays eggs is the Platypus. Turns out that was wrong. There are 2 mammals that do this. The platypus and the echidna - 4 if you count all 3 echidna types.
Platypuses also have a toxin spike in their back heel. Genome-ically, this is related to the toxin reptiles (like venomous snakes) have! More about Platypuses.
And we have learned more odd things from genome sequencing of these two mammals. It turns out that egg laying isn't something they learned (evolutionarily speaking) but something we other mammals forgot. In fact, echidnas can have either live babies or eggs - depending on the environmental conditions at birth!
One other thing they found, and they have ZERO understanding of why this might occur, platypuses glow under black light!
The world is far more complex that we think.