Ammonoids

Ammonoids are basically big shell-snails living in the dark caves of Mars.

Description
It is not uncommon for these creatures to travel in groups. Although rather big, they are also rather clumsy in combat and easily vanquished. The main problem is that they also attack with a rather nasty acid, which can really hurt and damage the equipment. Therefore, an attack from a distance with guns is the best way to deal with them effectively.

They can be found in the dark, Martian caves.