Stigorax

The Stigorax was a species native to Terra Alpha. The Seventh Doctor described them as "ruthless, intelligent predators". By the time of the rule of Helen A, there was only one left.

Biology
The Stigorax were rat-like carnivores. They fed on humans and were feared by the Pipe People of Terra Alpha. Despite this, Fifi was somehow tamed to be both a hunter and a pet by Helen A. (TV: The Happiness Patrol) This confused the Seventh Doctor, as he knew Stigorax to be among the most vicious predators in the universe. (PROSE: The Happiness Patrol)

History
The Stigorax were native to Terra Alpha. They hunted the Pipe People. When the humans came to Terra Alpha, they were almost wiped out. (TV: The Happiness Patrol) During the early years of the Happiness Patrol, Stigorax were given to its squad leaders, who used them to hunt killjoys. At Gladness Academy, a Stigorax named Fifi was given to an aspiring Happiness Patrol member named Gillian H. However, Fifi found Gillian to be a killjoy herself, and was then given to Helen H (later more famously known as Helen A). As Helen rose the ranks of the Happiness Patrol, Fifi obeyed her commands and was sent to hunt people who opposed her. (COMIC: The Death of Fifi) Fifi was killed while trying to hunt down the Seventh Doctor.

Before this, the last time the Doctor encountered a Stigorax was in Birmingham in the 25th century. (TV: The Happiness Patrol) When encountering Fifi, he was surprised because he believed Stigorax to be extinct by that time on Terra Alpha. (PROSE: The Happiness Patrol)

In invalid sources
A Stigorax was seen in Albert Square when the Rani trapped several incarnations of the Doctor in a time loop. (NOTVALID: Dimensions in Time)