Phantom fungus

Phantom fungi were a breed of predatory, mobile fungus feared by those underground for their natural invisibility.

"Phantom fungus — you get them in old tunnels and caves. It's a little one though. Full grown, it would have been chewing off Zuzzara's hip, not biting her ankles."

- The dwarf Mumchance.

Description
Ordinarily, the only details indicating the presence of phantom fungi were the sounds of soft squelching footfalls accompanied by a strange smell of mold. If rendered visible, phantom fungi appeared as a brown or greenish brown fungal mass supported by four stumpy legs. A cluster of nodules rested atop its main body and its gaping maw was lined with rows of teeth.