Giant koala

The giant koala (Phascolarctos stirtoni) is an extinct arboreal marsupial which existed in Australia during the Pleistocene epoch. Phascolarctos stirtoni was about one-third larger than the contemporary koala, P. cinereus, and has an estimated weight of 13 kg (29 lb), which is the same weight as a large contemporary male koala.

Although considered a part of the Australian megafauna, its body mass excludes it from most formal definitions of megafauna. It is better described as a more robust koala, rather than a "giant"; in contrast, a number of Australian megafauna, such as Diprotodon and Procoptodon, were unambiguously giants, even if not compared to their closest relatives.

The two koala species co-existed during the Pleistocene, occupying the same arboreal niche. The reason for the extinction of the larger of the two about 50,000 years ago is unknown, although there are various hypotheses for their extinction.