A marsupial is a type of lifeform with an external pouch used for incubating its young. Marsupials evolved on many different planets in the Milky Way Galaxy.
The Akritirian chaka emu was a hybrid marsupial from Akritiri in the Delta Quadrant. (ST reference: Star Trek Cookbook)
The Arcadians had a pouch in which their newborns spent the first three to six months of their lives. (TOS reference: Star Trek IV Sourcebook Update)
A landing party from the Earth Starfleet starship Enterprise identified several nocturnal marsupials on Archer IV in 2151. (ENT episode: "Strange New World")
The planet Boreth in the Klingon Empire was home to the bolmaq. (GKN novel: Enemy Territory)
On Earth, many types of marsupials were mammals that evolved in and around Australia, such as the kangaroo and koala. (TOS novelization: The Voyage Home; TOS novel: Death's Angel)
The Trill humanoids from the planet Trill were marsupials, and the dominant race on their world. There were four-legged animals on Trill that were also marsupials. (DS9 - Worlds of Star Trek: Deep Space Nine novel: Trill: Unjoined)