The Rot likely forces a hole in the corpse of its host, as the legs of the HLL appear to be coming out of a hole.

I think that the Rot isn't just something that takes over a living creature's body, I think it actually sucks the energy out of the host. This is likely the cause of Hunter's weakening after the end of Cycle 1, as Hunter doesn't have the energy needed to continue on and survive, unlike the other Slugcats. The Rot leeches energy off of the host in order to grow itself, eventually killing the host and using the corpse to grow, creating a Long Legs out of the corpse, and that Long Legs consumes more, eventually growing and becoming another colony of The Rot. And the fact that it is both a large, living creature and a minuscule parasite implies that any Long Legs might be able to split some of the smaller cores off of itself, and those cores will float away and inevitably end up inside of a living creature, eating away at it from the inside, and continuing the process.