Well, for sea animals Origami Sea Life by Montroll/Lang is a good choice. There are a few simple models in it, but most of them are intermediate to complex. The difficulty sounds in line with what you are looking for, with a lot of high intermediate/complex models.
For mostly (not all of them but mostly) mithycal creatures and models of 200 + steps, get Works of Kamiya Satoshi. A good review of it can be found at http://www.giladorigami.com/BO_Kamiya.html
I have Origami Sea Life, and made a few things that I like in the book. Chances are that I'll be able to put their photos up in a few days (if I can find the camera's USB Cord).
P.S.: I never noticed what Jonnycakes was talking about until yesterday!