Buffalo

Buffalo were cattle-like creatures that lived on Earth. Because Roz Forrester's family name, Inyathi, translated as "buffalo", she wasn't allowed to eat a buffalo. (PROSE: The Also People)

They were once the dominant species on Earth, but when their Herd-Leader was trapped in ice they degenerated without his mind. The Bovine Herd Leader revived thousands of years later and was able to travel back in time. He found his species were being wiped out by the Native Americans and guided Christopher Columbus to America in 1492, hoping the Europeans would wipe out the Native Americans and save his herd. However, the Europeans wiped out the buffalo themselves. (AUDIO: Trouble in Paradise)

The Seventh Doctor and Ace saw buffalo living in the Ngorogoro Crater in 1926. (PROSE: Prelude Birthright)

The last buffalo died in captivity in 2193. (PROSE: The Also People)

Cultural references to buffalo
The Sixth Doctor used the password buffalo  to access the UNIT telephone line. (AUDIO: Vampire of the Mind) The Ninth Doctor later told Mickey Smith to use same password to access a variety of areas of the UNIT website. (TV: World War Three)

Behind the scenes
The DWU fails to use buffalo as a synonym for bison — which it is, in a limited sense. Certainly, North American buffalo are bison. However, the kind the Seventh Doctor and Ace saw in Africa would have been and not at all bison.

Information from invalid sources
At different points, "bison", "buffalo", and "badwolf" are passwords to the Operations Board on the UNIT website. (NOTVALID: Secure Login)

The Ninth Doctor, after entering the password "buffalo" into the override code field, could navigate and fire a missile towards anywhere on Earth. (NOTVALID: Untitled)