The male platypus is one of only two known venomous mammals.

Actually while not common there are several venomous mammals including the Eurasian water shrew, Southern Short-tailed Shrew, Northern Short-tailed Shrew, Cuban Solenodon, and Haitian Solenodon. And of course the platypus.

Also there is the sort of venomous Slow Loris. These mammals have poison glands on their elbows which they suck on to get poison in their mouths subsequently delivering a poison bite.