Clyde is a tortoise previously owned by Len Pontecorvo. Sherlock Holmes and Len both visited the same conspiracy theory forums, so when no one had heard from Len, Sherlock decides to visit his house where he finds him dead. While Sherlock is looking for evidence, he finds Clyde and brings him home with him. From his sarcastic comments that tortoises make an excellent soup, Joan Watson initially thinks Sherlock intends to fatten and eat Clyde, but by the end of the episode it becomes clear that Sherlock is in fact fond of Clyde and never intended to harm him; he loves tortoises, remarking that "Clyde will likely outlive both of us."
Clyde is occasionally seen around the brownstone. He usually resides in a terrarium, but is often being used by Sherlock as either a prop in a crime-scene model or as a paperweight. He is immensely fond of lettuce and other greens, which are fed to him by Watson and Holmes.