- You may be looking for the Sontaran, Jingo Linx, also known simply as Linx or the 19th century spaceship.
Lynx was a cat generated by the Doctor's TARDIS, image-coded by the survival circuits as a projection when there was a dire threat. She was hinted at being made of artron energy and looked like liquid mercury because of her silver fur.
After the Process invaded the TARDIS, the ship collided with the Time Scaphe and reformed into a SARDIT. There, Lynx followed the Seventh Doctor and Ace through the wasteland until the TARDIS was restored. (PROSE: Cat's Cradle: Time's Crucible)
Because of the ordeal the ship's link with the Eye of Harmony was broken. Lynx accompanied the Doctor as he fought the Butler Institute on early 21st century Earth, while the TARDIS waited at his house. (PROSE: Cat's Cradle: Warhead) When the ship was finally repaired with protoplasm from Tír na n-Óg, Lynx returned to the mathematical cat's cradle within the time rotor. (PROSE: Cat's Cradle: Witch Mark)
She later followed the Doctor into Fred's dimension and helped Kadiatu Lethbridge-Stewart fight it. There was also a green cat which may have been the protoplasm that infected the TARDIS, as Lynx almost fought it. (PROSE: Transit)