Ankhegs are large (about 10' in length) insectoid burrowing creatures. Strange chirping noises may be heard right before an Ankheg emerges to attack. It is generally recommended that low-level parties use extreme caution in areas where they are known to thrive, and to fight only one or two at a time. They generally attack very quickly, with either melee or ranged attacks, as both contain acid, easily capable of doing ~20 damage per hit. When one is killed, it will drop some high-quality treasure along with an Ankheg Shell, which pointedly weighs 100 lbs.
They are primarily found in the Farmland Area North of the Friendly Arm, though will rarely appear on the outskirts of Gullykin and the Area North of Carnival. Like most mobs, their numbers are based on the strength of one's party, but it's safe to say that low levels should only encounter one per spawn, while a party averaging level 5 may encounter three, and only the most advanced parties will encounter more.
Baldur's Gate IIEdit
They will rarely appear in the Windspear Hills area, but likely alone. Along with a higher level party, they're no longer such a significant threat.
Close fast with Warriors and prioritize your targets. Try to have your best Warrior - particularly the one with the best savings throws and piercing AC - on point, but having at least one backup can be beneficial. Then the rest of your party can support with missile weapons and spells. Ankhegs are rather susceptible to magic, so most wands and offensive level 1 spells will be quite effective, though likely shouldn't be relied on - summoning spells being the most consistent. Otherwise, make use of magical equipment and potions as needed - similar to the occasional NPC confrontation, dealing with these beasts will likely require all the help you can get.
Assuming you find more than a few, there are two people you can sell them to - one within the city of Baldur's Gate (Chapter 5+), the other offering the creation of the unique Ankheg Plate Mail, which may prevent any future sales. In any case, you will have to haul the 100 lbs. stacks, which is when companions with high Strength can be really helpful, such as Minsc and Shar-Teel with their 320 lbs. and 250 lbs. carrying capacity, respectively. Assuming your PC is a Warrior class, they should also have superior Strength, being of even further use. In addition, there are various magical means at your disposal, such as a few spells or the Gauntlets of Ogre Power, either of which could likely help significantly. Lastly, each character may carry 20% more than their maximum capacity and still stay mobile, if significantly slowed.