Recently went down the rabbit hole (ok, I’m lying, I just read a Wikipedia article and one scientific study), and the venomousness of the platypus has crazy implications for mammal evolution:

It is thought to be an ancient mammalian characteristic, as many non-monotreme archaic mammal groups also possess venomous spurs.

(The monotremes are the platypus and the echinda, which unlike the platypus isn’t venomous but still has the spurs.)