Cthuldruches are actually just made of a layer of rubbery, stretchy skin, that gets completely slimy when they're horny. It's usually empty inside, unless they engulf stuff.
This one here just filled itself with air to be able to float.

I can't remember exactly everything I had in mind back when I created them (2009), but they were creatures that existed in four or five different colours, each one having some kind of special power/effect : the green ones had poisoned skin, for example.
They're playful creatures, but they're also incredibly dumb.

Normal state : http://www.furaffinity.net/view/7999684/

Also yeah, crappy background, but at least this time I've tried. <_<