The Cerunnos were a species of shapeshifters.
Biology
Cerunnos were creatures without a true form. They would constantly change form and shape, seemingly at random. They were also a very long-lived species, and could survive temporal damage without difficulty. The Cerunnos could also travel through time to a degree. Despite this, they had limited intelligence and were generally unable to recognise what was happening. (PROSE: ...And Eternity in an Hour)
After it was forced away from Alrakis, the Cerunnos created from Menkib was turned into an immaterial force and was forced to take a parasitic form. Though capable of shapeshifting, it preferred to take over people's minds instead. (PROSE: Moving On)
History
The Cerunnos had an unknown origin, and were thought to be from the Dawn of Time or possibly a collective nightmare. Many species of the universe had myths about them.
When Alrakis was affected by a temporal rift, the Yed-Prior Zaniah planned to use to make herself immortal and make her subjects live for only an hour. In order to test for this, Menkib volunteered to have his body destroyed and recreated with the temporal energy. The scientists were unable to recreate his morphic field, turning him into a Cerunnos. Though they could contain him, they were unable to change him back or communicate with him. When the systems started to break down, the Cerunnos broke free and started feeding on anyone who came close. When the labs were destroyed by a nuclear bomb, the Cerunnos was able to escape to Earth. (PROSE: ...And Eternity in an Hour)
After arriving on Earth, the Cerunnos suffered loss in both memory and power. It took the form of a human and wandered the Earth, trying to remember its past. Eventually, it planned to escape Earth and find the Doctor, who he blamed for his outcome. He took over the minds of several people in order to learn how to escape. He took over Jennifer Bendick to learn of human minds, Jonathan Kendrick to develop the technology to help him escape, and Scott Wojzek to form Tonska Industries. When Sarah Jane Smith was interviewing Scott, the Cerunnos learned of her past and decided to try to take over her mind. While she was asleep, he tried to contact the Doctor using K9 Mark III. Eventually, Sarah Jane was able to confront the Cerunnos and defeat him. (PROSE: Moving On)