There aren't alot of animals made in the Star Wars toy line. The Wampa, Taun Taun and Dewback have been done, but there are many more interesting species that can be added to your dioramas.
A great source of reference are the many books and magazines dedicated to the back history of the Star Wars Universe (such as the Star Wars Encyclopedia by Steve Sansweet, or The Essential Guide to Alien Species by A. M. Lewis, to name but a few). Even prototype or conceptual drawings are great references (drawings by Ralph McQuarrie or Doug Chiang) for species that never made it to the screen, or are so obscure they are hard to see in the movies. You can find pictures online or in the "Art of Star Wars" books. Two methods are basically used for creating these creatures. If you are lucky enough to find a similar toy or combination of toys, you can use that as a base for your beast. (ie. a dragon figure for your Krayyt). Similar techniques used in the 3-3/4" page on disassembly / re-assembly can be used on most figures. Use your imagination and a bit of sculpey, and "bam!" you have your creature.
Or, you may want to sculpt the figure from scratch. The biggest disadvantage to this is the figure will always be for show, not play. It is extremely difficult to get any true articulation from sculpted figures. If you want to try your hand, I have found that an armature of aluminum foil is a good starting point. Just like sculpting anything else, you have to have patience and experience to do this right. You'll also need to look around for tools to sculpt with - nut picks, needles, wooden tools, even old dental tools are often used. Also gather items that you can press into the sculpey to make textured surfaces ... I've used Bossk's leg to press into the skin of my Krayyt Dragon (seen above during the sculpting phase) to get the effect of scaley skin. Sculpey rolled between two terry towels or woven fabric, then chilled before removing, also makes great textures.
Again, your success in recreating a good beast is to get as many reference pictures as you can. As well, make sure you have your beast in the right scale (especially if you are placing him in your 3-3/4" dioramas).