Dynamite comes in small packages! Gremlins are vicious beasts with loads of teeth and claws, wrapped in a bad attitude. Rarely found alone, Gremlins usually travel in packs and attack like a swarm of bees.
The Gremlin, introduced in Scourge of Armagon, is a weak Enemy with the unique ability to steal almost any firearm that the player has out and use it against them. The weapon can be taken back when the thief is killed and it drops the gun.
Gremlins are crafty little devils, but they do have their limits. A Gremlin will not steal your Mjolnir, standard shotgun, or axe. By switching to one of these weapons, he won't be able to steal your weapon.
—Scourge of Armagon Manual
The Gremlin can steal almost all weapons except the Shotgun, the Axe, or the Mjolnir, and will use it against somebody. When killed, the Gremlin will drop the stolen weapons in the form of a Backpack that the player can pick up.
The Gremlin can stop attacking the player and start attacking a monster corpse. If the player lets the Gremlin do this for too long, only the head will remain of the corpse, and another Gremlin will spawn. It's possible that Gremlins reproduce asexually and, after having enough biomass, they give birth to another Gremlin.
The Gremlin resembles a Fiend but it's gray, has yellow eyes, and is smaller. They can attack by leaping at somebody and clawing. While not particularly dangerous on its own, their firearm-stealing ability can be a pain if one is distracted by more dangerous foes.
The Gremlin should be killed fairly quickly, because running from it gives it the opportunity to build up a small army if a lot of dead monsters corpses are around.
Gremlins are very slow both moving and clawing. Alone or in small groups they can be safely fought even with Axe.
Always equip your weaker weapons, like the Nailgun, against the Gremlin. If it gets your Rocket Launcher or Thunderbolt, it can destroy you in a matter of seconds.
Alternatively, equip one of the aforementioned weapons the Gremlin can't steal if you're within jumping distance of one.