Clyde

Joan: "Why does Clyde have tape on his back?"

Sherlock: "It's a cross. He's an ambulance."

Clyde is Sherlock Holmes' pet tortoise.

Holmes rescued Clyde from the home of a murder victim, initially telling Joan Watson that he was going to fatten him up and eat him. However, he eventually revealed he was joking and appears to have kept Clyde as a pet (and occasional test subject for experiments).