Would it be adventageous for a mammal to be poisonous?

It might have been advantageous for mammals to have the same adaptations as rattlesnakes. Then again it might have been advantageous for rattlesnakes to be warm-blooded and have legs. Each organism is well adapted for its own particular needs. I can't think of any mammalian predator that is going hungry because it doesn't have venomous fangs.