Maw

Maws are large creatures found in humid areas such as Pools and Marshes. They are worm-like in appearance, but also have two large scythe-like arms. Maw have a large amount of health.
They are immobile, but can continuously root anything that comes close to them. 
Though Immobile, after a Maw hasn't aggro'd anything for a while it spins and disappears underground and reappears somewhere else in the area.
(presumably the inactivity timer is somewhere between 4 and 8 minutes, but this needs confirming)