Tuval

Tuval was a Zygon scientist.

After their homeworld Zygor was destroyed, he and a small group of Zygons crashed on Earth around the 17th century and were stranded. He and other Zygon scientists worked on a breeding program to create large quantities of Skarasen, which they perfected by the end of the 19th century. They were going to be used to destroy humanity so it could become the new Zygon homeworld. The Eighth Doctor stopped the plan and took Tuval and the Skarasen to another planet to settle peacefully. During much of his interactions with the Doctor, he assumed the form of his companion Sam Jones. (PROSE: The Bodysnatchers)