All hail the rat king! A rat king is a hellish entanglement of rats that are bound together by their tails. This can happen for different reasons, such as their tails getting tangled into something like hair or string, or a sticky material such as sap or gum. Although there are some preserved rat kings that you can go see in museums, there is no proof that these rats are not a hoax, tied together by a human.
From Wikipedia on the skepticism surrounding rat kings:
Zoologists remain skeptical of it, saying that, while theoretically possible, the rats would barely be able to survive in such a condition for a long time. Particularly if the temperatures rose or if they bite their own or another one's tail to try to free themselves. They would also be in pain. Since black rats do cluster together during winters for warmth, it could be remotely possible for it to be naturally occurring. Regarding some of the preserved specimens being fakes, the fabrication would have most likely been done on dead ones, given how difficult it would be if the rats were alive. Proper observation of the phenomenon, under scientific scrutiny, occurring naturally with live rats from the start till the end, has yet to happen
There are very few reports of rat kings being spotted in modern times. Part of me feels quite relieved about the idea that rat kings are a mere hoax, but the other part of me feels a demented sense of hope to stumble across a giant rat king in real life someday.