It's double abdomen thing fooled me into thinking that it was a third leg, until I saw the title. There's another way to come up with ideas. Start thinking of random stuff in your sleep. Sooner or later, your thoughts will switch tracks (as is my case last month. Now I just need fifteen dollars so that I can have help bringing my kaiju idea to life)
Basically, if you don't get the a clear vision in you mind, just throw out random brush-strokes. Sooner or later you will see strange creatures in there (just as most people do in clouds). Then it is simply a matter of polishing, and testing various options.