Slimeslug

The Slimeslug is a mutant mentioned in the Metro 2033 Novel.

Overview
During the novel's chapter, 'There Up Above' (English Version), Artyom is travelling to Smolenskaya alone, after Melnik is forced to take a wounded Ten back to Polis; after being attacked by Librarians in the Great Library. The only advice Melnik gives Artyom is to stay away from the middle of the road, but also to avoid going too close to abandoned houses, and to make sure he makes it to Smolenskaya by sunrise, lest the mutants that come out during the day attack him when he is weak (he would be blinded by the sun).

On the way, Artyom notices mutants (most likely Watchers) following him. Despite shooting near them to frighten them away, more and more stalk him from the abandoned houses. However, they are scattered after a Demon carries one off, but they do not hide for long. To avoid being attacked, he disobeys Melnik's advice and seeks shelter inside an abandoned apartment building. The Watchers do not follow.

Artyom makes it to a room and locks himself in. The Watchers on the street below can still see him, so he shoots one with his Kalash. This scares them away, but Artyom waits to be certain they will not return. While exploring the apartment, he finds a picture entitled, "Little Artyom is 2 years and 5 months." Thinking this is his mother, Artyom forgets himself and lingers too long. Looking out the window, he realizes that it is almost light and he must hurry if he is to make it to Smolenskaya before sunrise.

Opening the door, he discovers that there is a trail of transparent slime running down the stairs, though, thankfully, it has not approached his door. Fearing the creature that produced it is nearby, he leaves the apartment building and makes his way to Smolenskaya, avoiding being attacked by Watchers.

Saved by Melnik, in the Smolenskaya hospital, Artyom tells Melnik of his story. Melnik complains that Artyom is almost two hours late. However, when Artyom tells him of the slime on the stairs, Melnik gets annoyed, chastising Artyom that he went into a building, saying that the creature could have dropped in on Artyom at any time. He asks if Artyom stood in the slime, but is not convinced, so checks Artyom's boots. However, he is satisfied and moves on to more pressing matters.

When leaving the hospital, Melnik gives Artyom new clothes, saying that his old ones will be cleaned and disinfected, most likely just because of the radioactive conditions on the surface, but possibly because Melnik is still not certain that Artyom did not step in the slime.

Trivia

 * It's unknown what produces the slime, what it looks like, and why the slime is so dangerous. Still, both the Stalkers and the Watchers fear whatever it is.
 * It's possible that Melnik was simply warning Artyom to avoid buildings, because they are dangerous, not specifically because of the Slimeslugs - in either case, abandoned buildings are very dangerous in the post-apocalyptic world.
 * Slimeslugs sound similar to a Metro 2033 Video Game only creature, the Giant Ameobae, and the latter may be based on the former.