Oxy-Leeches

The Oxi-Leech of Mars feeds on the radioactive material Oxium. The leech itself -despite being proably one of the most disgusting sights on the planet- isn't a threat in combat. It does next to no damage and is easily killed. It's true danger lies in that it snatches and devours the needed Oxium at an alarming rate, leaving the adventurer with bare hands, since Oxium is needed to run man Martian machines.

Therefore kill them from a distance with maximum prejudice.

Lore
''This slug-like underground life form feeds on oxium. Unfortunately, these primitive creatures make no distinction between veins of oxium ore and travelling adventurers with oxium in their packs. The oxy-leech has a hideous, toothy mouth opening, but does very little physical damage, preferring to steal oxium and flee before victims can react. Generally found near cave entrances and in ravines, the oxy-leech is a disgusting creature. but more a nuisance than a genuine threat.''

- from "Mysteries of the Red Planet", Martian Dreams