Plenty of animals have their own poison, but the African Crested Rat is the only known mammal outside of humankind to create poison from an external source. The rat chews on the bark of the Acokanthera tree until it starts to ooze a poisonous goo; the rat then rolls around in the substance to protect itself from any potential predators. The animal's hairs have many tiny holes, allowing it to absorb the poison into its fur for later use.
Humans from the area are also known to use the Acokanthera tree for a similar purpose—poisoning their arrowheads. Unlike the humans who use the tree, though, the crested rat is immune to the poison. (Researchers still aren't sure how this is possible.)
[Via Discovery News]