Ankheg (CR 3)

Large Magical Beast
Alignment: Always neutral
Initiative: +0; Senses: darkvision 60 ft., low-light vision, tremorsense, Listen +4, and Spot +3

AC: 18 (-1 size, +9 natural), touch 9, flat-footed 18
Hit Dice: 3d10+12 (28 hp)
Fort +6, Ref +3, Will +2
Speed: 30 ft., burrow 20 ft.
Space: 10 ft./5 ft.
Base Attack +3; Grapple +12
Attack: Bite +7 melee
Full Attack: Bite +7 melee
Damage: Bite 2d6+7 plus 1d4 acid
Special Attacks/Actions: Improved grab, spit acid
Abilities: Str 21, Dex 10, Con 17, Int 1, Wis 13, Cha 6
Special Qualities:
Feats: Alertness; Toughness
Skills: Climb +8, Listen +4, and Spot +3
Advancement: 4 HD (Large); 5-9 HD (Huge)
Climate/Terrain: Warm plains
Organization: Solitary or cluster (2-4)
Treasure/Possessions: None

Source: Monster Manual

Improved Grab (Ex): To use this ability the ankheg must hit with its bite attack. If it gets a hold, it deals automatic bite damage each round the hold is maintained. If the ankheg is damaged after grabbing its prey, it retreats backward down its tunnel at burrowing speed, dragging the victim with it.

Acid (Ex): Acidic enzymes drip from an ankheg's mouth each round it maintains a hold. It automatically deals 1d4 points of acid damage each round in addition to bite damage.

Spit Acid (Ex): Stream of acid 5 feet high, 5 feet wide, and 30 feet long, once every 6 hours; damage 4d4, Reflex half DC 14. One such attack depletes the ankheg's acid supply for 6 hours. It cannot spit acid or deal acid damage during this time.

Ankhegs do not use this ability unless they are desperate or frustrated. They most often spit acid when reduced to fewer than half their hit points or when they have not successfully grabbed an opponent.

Tremorsense (Ex): Ankhegs can automatically sense the location of anything within 60 feet that is in contact with the ground.

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.)

Clusters of ankhegs share the same territory but do not cooperate. If several attack, each tries to grab a different foe. If there aren't enough targets, two might grab the same creature in a tug of war.