There are only 2 main species of mammals which are venomous by nature. These mammals include the shrew and the platypus.
Shrews are small, superficially mouse-like mammals of the family Soricidae. Although their external appearance is generally that of a mouse with a long nose, the shrews are not rodents and not closely related and the shrew family is part of the order Eulipotyphla. Shrews have feet with five clawed toes, unlike rodents, which have four. Shrews are also not to be confused with tree shrews, which are also unrelated, and belong to their own order, Scandentia. Shrews are distributed almost worldwide and of the major temperate land masses, only New Guinea, Australia, and New Zealand do not have native shrews at all; South America has shrews only in the far-northern tropical part.
Certain species of shrew are venomous, as in these species produce a toxic secretion which are channeled via little grooves in their teeth on the outside that the venom follows into a bite wound, which probably helps to immobilise especially large prey, such as other small mammals, reptiles, amphibians, fish and larger invertebrates, by its effects on the nervous system. This poisonous bite has been reported from North American Blarina and European Neomys. The bite can be quite painful to a human hand for, although shrew's teeth rarely puncture the skin, the toxin in the saliva of some species seems to produce a slight inflammation and reddening of the skin which can persist for several days.
The platypus (Ornithorhynchus anatinus) is a small, semi-aquatic mammal endemic to the eastern part of Australia, and one of the four extant monotremes, the only mammals that lay eggs instead of giving birth to live young. Please kindly note that the other three are from the echidna family. The scientific name Ornithorhynchus is derived from "ornithorhynkhos", which literally means "bird nose" in Greek, and anatinus means "duck". The name platypus is often prefixed with the adjective "duck-billed" to form duck-billed platypus, despite there being only one species of platypus.
All platypuses are born with spurs on their hind feet to inject venom. The spurs on the female fall off after the first year. The spurs on males become venomous during the breeding season.
N.B. Apparently, there is a slight difference between being vemonous and being poisonous. The definition hinges on how the toxin gets into the prey. A poison is something that has to be eaten or breathed in for it to take effect, that is ingested or inhaled. A venom is usually harmless if eaten. In order for it to do harm, it must come into contact with tissue underneath the skin, that is injected.