Gather 'round, traveller, for an unusual sight!
The Doctor is a naturally timid creature. It leads a solitary life, avoiding both those of other species and those of its own kind. It tends to live in a small warren in a dark location, as the day star burns its thin hide.
Due to its small stature and limited intelligence, the Doctor is vulnerable to a long list of predators. Its only natural defense is its ability to squeeze into small spaces and blend in (somewhat poorly) with its surroundings.
Much like a mouse who defecates on every surface it touches, the Doctor coats its warren in a thick layer of yarn. For purposes scientists have been unable to determine, it uses this yarn to create scarves, hats, plush toys, and the occasional blanket. Like a panda, the Doctor would far rather eat plants than mate. It's a wonder the species has not yet died out.
If you ever see the Doctor in the wild, do not approach it, as it will likely die of fear if it realizes it has been spotted. If you stand still, it may eventually approach you slowly and cautiously. As it becomes more comfortable, it may be possible for you to reach out and gently touch it on the head. It would be most pleased if you asked it to make you something out of its vast warren of yarn. In fact, creating things for others is the Doctor's sole joy in life, and it would really appreciate it if you allowed it to make underpriced yarn goods for you.