Symbiote

A symbiote is a fictional living sentient, alien being that bonds with other living organisms, usually in order to survive (the term "symbiote" refers to the way that the alien being lives in a symbiotic relationship with its host). The concept was first originated by Marvel Comics, particularly in the villainous character Venom, and is often synonymed with the term "living costume" due to the way that such beings envelop their hosts.

Anthropomorphic symbiotes enjoy a fair degree of popularity in the furry fandom, particularly in the rubberfur and transformation subfandoms, and are often the subject of pertinent art and literature. Symbiotic transformations, which result in a stark alteration of the physical structure of the host, may usually be depicted as orginating from either from some sort of amorphous, shapeless goo or from a costume that is seemingly meant for more casual fitting (i.e., a fursuit) but instead bears a hidden symbiote that bonds with the first potential host within an approximate range.