The Altarian Marsupial is a small, presumably mammalianoid creature and one of the many live organisms kept by Doctor Phlox in his sickbay. It seems to be quite aggressive, as when Captain Jonathan Archer holds a box containing the marsupial, Pholx advises him to be very careful. The box then shakes violently while the creature releases a high hissing sound. When Phlox imitates this sound, the marsupial seems to calm down.
According to Phlox, the Altarian Marsupial's droppings have the highest concentration of regenerative enzymes found anywhere, which is probably the main reason why he keeps one.