(yuh-POK) n. [After the Oyapock, a river in South America.] An aquatic marsupial mammal, Chironectes minimus of tropical America, with dense fur, webbed hind feet, and a long tail.
Sometimes also referred to as a water possum. Both sexes have a pouch. As with other marsupials, the female houses her young in her pouch. She can swim with them in there. The male pulls his external genitalia into his pouch when swimming or moving swiftly. The yapok's tail is prehensile but too thick to be of much use. The yapok swims with its forepaws extended to grab prey (it's carnivorous), paddling with the webbed hind feet.
There's a picture in my dictionary, but I couldn't find an electronic image to share.