I always though the cacodaemon looked rather silly in the original Doom. In Doom 3 the design was a lot better, but they just ended up beign rather unremarkable, although unnecessarily hard to kill, because most longrange weapons in Doom 3 were borderline useless.
The yellow bloated things are supposed to be some sort of floatation bladders, that are full of gas and whatnot, and popping all three drops this sucker into the ground. It will then proceed dragging along the surface with it's tentacles, blasting electric-bolts and whipping at the player mercilesly.
What monster to do next... Suggestions?