Slime slug image
The slimeslug is a type of mutant briefly mentioned in the novel Metro 2033. Not much is known of these creatures. They are considered a threat by the Stalkers (specialists who travel between the metro tunnels and the Moscow ruins), as shown in the chapter "There Up Above," where the Stalker leader Melnik warns Artyom to stay close to, but not to enter buildings. Artyom's closest encounter, and so far the closest any character in the Metro series has gotten to a slimeslug, had him trapped in a room, picking off watchers. When he left, he saw a trail of slime outside. This slime is also believed to be dangerous, although how remains a mystery.