Legend of Grimrock has many different kinds of monsters, with each type having unique powers, strengths, and weaknesses. 

The descriptions below were taken from the game's manual:

GIANT SNAILS. These overgrown snails can be encountered in the upper levels of Mount Grimrock. They are quite slow to move and react but don't let it deceive you, their acidic bite can sting an unwary prisoner badly! A gently fried snail slice is absolutely delicious with a pat of butter.

UNDEAD SOLDIERS. The spear wielding soldiers who guard Grimrock are now long perished but that does not stop them from fulfilling their oath to protect the place from all harm. Undead soldiers are usually encountered in military units and they are equipped with large shields that protect them from damage. As they don't have internal organs and they don't need to breathe, poison is not an effective weapon against them.

HERDERS. These neutral creatures have lived in Grimrock for a very long time, perhaps making them the original occupants of the place. They live in a strange symbiosis with plants inside the mountain. Usually they keep to themselves and are peaceful, but if their kin or any plants are in danger they get enraged and even sacrifice themselves if they can save some plants by doing so. Herders come in many shapes and sizes. The large ones defend themselves by slamming with their powerful tendrils while other kinds use poison.

CROWERN. If you see a strange winged being resembling a crow and a wyvern, you have encountered a Crowern. No doubt their origins lie in some dark laboratory experiment. Crowerns are strong and very aggressive and they use flight and speed to their advantage in combat. There may be evolved forms of Crowerns deeper in the dungeons.

UGGARDIAN. Uggardians are spirits that have been summoned from Outer Realms to guard important places such as tombs and vaults. They resemble heavily armored warriors whose bodies are made of fire. Anyone standing next to a Uggardian can get blistered very quickly!