Panther
Panthers were a type of felid mammal the Third Doctor and the Brigadier once encountered at a zoo, early in the Third Doctor's life. Normally fierce, this panther was as docile as an ordinary house cat. This odd behaviour made the Doctor and UNIT suspicious of nearby activities. (TVC: The Arkwood Experiments)
The Second Doctor, John and Gillian also encountered a panther. Once, while visiting New York City, the trio went to a zoo and ran into a panther that was being controlled by a misguided human scientist. The Doctor was only able to avert the panther's unnaturally aggressive attack by using acid against it. (TVC: The Monsters from the Past)
On Christmas Eve 1941, the Eleventh Doctor claimed there were panthers on an upstairs floor of Uncle Digby's house. (DW: The Doctor, the Widow and the Wardrobe)
Behind the scenes
In reality, "panther" is an ambiguous alternate name for several types of big cat, including the leopard, cougar, and jaguar. The text of neither The Arkwood Experiments nor The Monsters from the Past help us understand precisely what the Doctor encountered. However, as drawn both "panthers" appear to be black panthers. Again, though, a "black panther" isn't technically its own species, but merely a type of leopard, jaguar or cougar. As far as the DWU is concerned, however, panthers have been portrayed as a unique species.
