The second time I used these (a few years later), the party was looking for lost children to rescue. They'd heard movement or indistinct voices a couple times, and a couple other times they'd opened silent wardrobes or drawers only to have these critters jump out at them. Eventually, the party got strung out down a hallway: a decoy fight at one end, an ambush through an open doorway in the middle, and two kid-sized lumps hiding in a pile of laundry at the other end. One PC defends the laundry pile; he hears voices from it. "Yeah, what is it, kid?" he asks over his shoulder. The response is indistinct, so he glances back, and it isn't kids it's two of these critters with laundry on their heads mimicking kid-sounds with their mandibles.
It worked so well these first two times that I can't resist trying again whenever enough new players cycle in. It's how the the larval tank beetles in my second Pathfinder run were supposed to play.
I think next time, I'll have the players trying to meet or find an NPC, but everybody in the NPC's location has evacuated to certain well-known mines or tunnels. The PCs come alongside a deep shaft, where a small figure in a (blood stained) cloak clings desperately to a chain hanging out of the party's reach, with a heavy metal elevator or slab or something sitting against one of the other walls, too high to be useful. If (when) a PC jumps or falls onto the free hanging chains, weights will shift; the heavy metal will rise, unsealing a tunnel and freeing the swarm of critters therein; and the cloaked figure - another of the critters - will attack the PC as their chains sink into darkness.
Shriekipedes, Centipede Mimics or Jack in the Box Bugs
Never had a good name for these critters.
For D&D5e, I think I'd start with "Giant Spider" as a template,
[Easter Edit: CON 6, HP 12, AC 17? In our homebrew they were agile and strong-shelled, but couldn't survive more than one or two typical hits. Not sure how to translate that to 5e. /Edit]
Lurking: these things are three to seven feet long, can fit through any hole a typical dog can, and tend to trap themselves in chests and cupboards; they're great at pulling lids or doors shut but crap at pushing them open again. They're more likely to chew or dig their way out through a back corner.
Face Grabbing: PCs hate having stuff latched onto their faces, so that's what these things go for. If the attack succeeds, the PC is likely blind and/or suffocating; if the attack is stopped by a helmet, there's a good chance the helmet will be pulled off; if the attack misses, the bug might latch onto a nearby wall or something by accident.
Backwards and Forwards: the head and the tail are difficult to tell apart, and they're both good at grabbing stuff. When one side latches onto something big (like a wall or a medium size creature), the other end gets advantage on strength checks. The bug can't voluntarily let go of something without making a DC 10 INT check.
Shrieking, 3x per long rest to: cast Counterspell or Dispel (with a +3 ability modifier), combo with another bug's bite attack to count as a magic weapon with the sonic damage type, add d8 Bardic Inspiration on the next bug action against a chosen target, inflict d8 Bardic Disinspiration on the target's current action... other sonic effects aren't out of the question.
I don't know what the "challenge rating" for these would be. Low, I imagine; I think I tend to fall back on them where other people would be falling back on basic skeletons or zombies.
[Easter Edit: I forgot! Because our high school group had so many mages, these were highly resistant to magic. For 5e, I'd give them advantage on spell saves, and if they get 20 or more on the save or counterspell roll, the spell reflects back on the caster.
...their "challenge rating" might be higher than I think.
I like how jump-scare monsters can make the players paranoid. I try to prime them before the quest to consider half-heard noises nonthreatening, and by the end have them paranoid enough for friendly fire against already injured NPCs doing their best to hide from the monsters. I sometimes also try to deescalate their paranoia afterwards, but rotating GMs from week to week makes that less of a factor. /Edit]
PS: Happy, hoppy Easter Eve?
