An alien parasitoid macrovirus.
The adult aliens resemble deformed potato beings with legs,
while the younger aliens - nicknamed "shit-weasels" because they can
be created in a host organism's stomach and escape by eating their
host's body between the stomach and anus - are legless, smaller
versions of the adult alien.
Both adult and young aliens have a mouth consisting of a slit on the
underside of the head that goes down the length of the worm.
The lips separate to reveal hundreds of teeth that can bite through steel.