Rat kings

Seen here is an example of a purported rat king, a giant rat beast created when many rats get their tails tangled together. Legend has it that the rats then grow together into a single creepy entity. This mummified "rat king" was discovered in 1828 in Buchheim, Germany and is currently on display at the museum Mauritianum in Altenburg, Germany. From Wikipedia:

 Wikipedia Commons F F4 Ratking
The earliest report of rat kings comes from 1564. If real, the phenomenon may have diminished when the brown rat (Rattus norvegicus) displaced the black rat (R. Rattus) in the 18th century. Sightings have been sporadic in the modern era; most recently comes an Estonian farmer's discovery in the Võrumaa region on January 16, 2005.

Most extant examples are formed from black rats (R. rattus). The only find involving sawah rats (Rattus rattus brevicaudatus) occurred on March 23, 1918, in Bogor on Java, where a rat king of ten young field rats was found. Similar attachments have been reported in other species: in April 1929, a group of young forest mice (Apodemus sylvaticus) was reported in Holstein; and there have been reports of squirrel kings. The Zoological Institute of the University of Hamburg allegedly owns a specimen.

Rat kings are not to be confused with conjoined twins, which arise in many species. Rat kings would grow together only after birth.