Nudibranchs (noo-dee-BRANKS) are a type of sea slug. Lacking a shell, most of them eat poisonous anemones and other poisonous things and incorporate the poisons into their own bodies. They’ve evolved very striking coloration to warn off predators (“bite me and you die”). They’re generally scavengers and bottom feeders (we’re talking about slugs, it’s not like they can chase down anything faster than … another slug).
National Geographic Magazine did an article about them and they put it up on their website, complete with some really amazing photographs, check it out: link
What does this have to do with dungeon ecology? As a guy with a science background, I like to incorporate elements of realism in my dungeons to make them more lifelike. Sometimes this means a nearby group of prey animals or that predators can cull. Sometimes it’s a simple water system or waste-disposal location or creature. And sometimes it’s nonaggressive creatures that serve an ecological purpose and aren’t a threat to adventurers.
For dungeons, it’s easy to drop an otyugh into a pit and call that a “bathroom,” but an otyugh is also just another excuse to give PCs XP. What if your waste-disposer was instead a slug as big as a dog? Normally it feeds on the mildly-toxic glowing lichen clinging to the dungeon’s walls (concentrating it within its own body), but given a supply of nutrient-rich poo it contentedly munches on that and doesn’t wander much. And the locals leave it alone because its skin is covered in countless tiny poisonous stingers … but every now and then a tribe nets one, drags it into a pot, and boils it down into an icky paste, which they put on their weapons….
So you end up with a creature that looks interesting, isn’t aggressive, it isn’t a pet creature, and probably just sits there while the PCs fight, rest, etc. They’ll wonder if it’s a trap, or a spy, or something weirder. Eventually they’ll realize the local inhabitants don’t pay any attention to them (even in the middle of battle) and they’ll just be living “dungeon debris” … perhaps something that’s an obstacle in the middle of battle (“don’t step in the slug’s square, you might get stung if you bump into it”) but not part of the kill-xp-loot cycle.
Anyway, check out the pictures, they’re awesome.