John has a secret: he's a werewolf. Sherlock suspects something, because occasionally (not necessarily when the moon is full, it's too cliché :P. I prefer John able to be at least mostly in control of the entire thing) John has to let the wolf free (he gets out of the city and hunts animals in a forest). Sherlock knows that John lies when he says he has to leave for a couple of days for one reason or another. But werewolves don't exist, right? Sherlock is very curious and it takes him a lot to allow John to keep his secret to himself (not that he doesn't try to deduce anyway).

One day they get into a big trouble. They are surrounded by enemies, who want to kill them, and there's no chance for John and Sherlock to survive. John knows he has no choice and he would never let Sherlock get hurt, so he turns himself into a big wolf (or something very wolf-like) with thick, light fur (like his hair). The wolf is extremely fast and powerful. He tears everybody and everything that can be a threat to Sherlock and himself apart.

Sherlock is shocked (how can he not be?). When the wolf approaches him, he thinks it's going to kill him, too, not sure if John can control himself in this form. Instead, John whines and licks his hands lovingly and sniffs him all over to make sure he's not hurt.

John/Sherlock, please. I prefer always top!John.

Bestiality is not really my thing, but I don't mind John having fun as a wolf, too, as long as it's consensual (an experiment, obviously ;) ).