Inspired by a picture of a rather beautiful, if not deadly, sea slug that I saw today.
Giant Sea Slug (Sea Swallow)
Medium Vermin, Neutral, Non-Intelligent; Solitary
Hit Dice: 6
Armor Class: 13 
Attacks: 4 appendages (1d4 + poison), bite (2d4)
Move: 20  (Swim 5 )
Saves: Fort 10, Ref 12, Will 12
XP: 600 [Expert]
Giant sea slugs of the sea swallow variety are quite beautiful, but very deadly. They can float upon the surface of the water, upside down, due to gas sacs in their bodies, or crawl on land (though always in damp places ... like dungeons, for example). They prey on larger creatures (monstrous jellyfish, giant slugs, pirates) and will prey on one another as well. They store the poison of creatures they eat in their bodies, mixing up powerful toxins that they use to kill their prey.
Special Qualities: Blindsight, immune to poison