Fapping Spiders are an imported enemy (from Penumbra: Overture) used in the Amnesia Mod: Tenebris Lake. They got their name due to the noise they make when moving around.
Pewdie named them 'Jens' during the episode 7 of the Amnesia Custom Story "Tenebris Lake".
They lunge at Pewdie when they get near him. On one occasion when Pewdie was running away from the monster, one of the spiders jumped at him and scared him. The monster got the chance to get close to Pewdie and killed him.