Mittens was a cat belonging to Jenny Flint.
Mittens had gotten stuck in a tree, so Vastra asked Strax to get her out which he did with his Time Shark. He rode the shark through the time vortex to stop a squirrel from planting the acorn that would become the tree. He found the squirrel which planted the acorn and the shark ate it. Back in Victorian London, the tree disappeared beneath Mittens and she fell into Jenny's hands. (COMIC: The Adventures of Strax & the Time Shark)