A wellspring is a valuable thing within the dry stone corridors of Grimrock. However, while there is a large basin of water near the center of the spire, it came with some rather nasty surprises. And those nasty surprises were grown and mutated by the magicks of the prison, soon growing, much like the spiders, into giant beasts with very tough shells and more than a little aggression to throw into any adventurer's face...
Giant Crabs are, true to their name, literally massive crabs. Stalking the halls of Mount Grimrock's lower dungeons, they are slow, but more than happy to feast on the remains of dead adventurers. Their thick carapace means that even a single one can take a lot of damage before falling in battle. Unlike most other monsters, they are able to strafe to the side, and may catch adventurers off guard with a sudden attack.
The creatures return in the sequel, mainly populating the Ruins of Desarune and its lower floors. One in particular wanders the halls of the Lexiconary, carrying a Round Key that opens one of the floor's secrets.