I am reading an article on deadly creatures and I had ideas to steal.
1. One of the creatures was the box jellyfish. While reading, I had the idea of a floating, jellyfish-like creature in a dungeon (like a grell) with poisonous tentacles that dangled beneath it. But then I thought, "just use a grell." Then I suddenly had the mental image of a jellyfish "swimming" in the dungeon corridor like a blooper in Super Mario Bros the Lost Levels. Despite the complete lack of water, that creature (and likely other aquatic menaces) "swim" around the dungeon. Perhaps they are ghosts. Perhaps there is a slight overlap with the Elemental Plane of Water.
2. An earlier article about other deadly creatures featured the huge swarms of giant jellyfish plaguing Japan. The article mentioned that these swarms arose out of nowhere and that killing the jellyfish is useless because they release millions of eggs/spores that can mature into jellyfish when they die. Also, these jellyfish weigh 440 lbs. Whether actual jellyfish in the ocean or similar creatures who float along on land, this is scary.
3. Yet another aquatic creatures, the blue-ringed octopus looks cool and reminds me of a tako from Oriental Adventures. I like the idea of an unusual marking to warn PCs to stay away. I also like the fact that they are the size of golf balls and paralyze a victim of their bite (which is strong enough to penetrate wet suit gloves).
4. There was also mention of the deathstalker scorpion, one of the most deadly scorpions known. Good reference for a black scorpion.
5. Another article mentioned the Malaysian/Bruneian ant, whose mandibles contain large sacks of poison. When threatened, the ant detonates these sacks, spraying poison at its enemies while also causing its own head to explode.