The ankheg is a burrowing monster with a taste for fresh meat. An ankheg has six legs, and some specimens are yellow rather than brown. It is about 10 feet long and weighs about 800 pounds.
An ankheg burrows with legs and mandibles. A burrowing ankheg usually does not make a usable tunnel, but can construct a tunnel; it burrows at half speed when it does so. It often digs a winding tunnel up to 40 feet below the surface in the rich soil of forests or farmlands. The tunnel is 5 feet tall and wide, and from 60 to 150 feet long ([1d10 + 5] x 10).

An ankheg usually lies 5 to 10 feet below the surface until its antennae detect the approach of prey. It then burrows up to attack. (Treat this as a charge, even though the ankheg does not need to move 10 feet before attacking.)
Clusters of ankhegs share the same territory but do not cooperate.
Improved Grab (Ex): To use this ability, an ankheg must hit with its bite attack. It can then attempt to start a grapple as a free action without provoking an attack of opportunity. If the ankheg is damaged after grabbing its prey, it retreats backward down its tunnel at its land speed (not its burrow speed), dragging the victim with it.
Spit Acid (Ex): 30-ft. line, once every 6 hours; damage 4d4 acid, Reflex DC 14 half. One such attack depletes the ankheg?s acid supply for 6 hours. It cannot spit acid or deal acid damage during this time. The save DC is Consitution-based.
An ankheg does not use this ability unless it is desperate or frustrated. It most often spits acid when reduced to fewer than half its full normal hit points or when it has not successfully grabbed an opponent.

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System Reference Document -> List of Creatures