Monotreme (Mammals that lay eggs)

The weirdest mammal clade: they lay fucking eggs. Only 5 known species alive as of 2020.

Eggs are basal: they simply didn't evolve out of what other reptiles do. From which we conclude that milk came before eggs stopped.

So this is the most basal subclade of mammals.

Etymology: means "single hole" in Greek, because like other reptiles it has a single hole for shit, pee and fucking: the cloaca.