If you want to buy premade animal models then I too can recommend Lyne's Creations store which is mostly dedicated to Poser animals. And don't forget DAZ of course. If you want to actually model your animals, that's entirely a different story. You can certainly model some animals with Xfrog 3.5. The best candidates are those animals who have a simple body mass and many appendices, such as spiders, bugs, viruses (yep!), some fish. For animals that have articulated body mass (such as humans, felines, dogs, etc.) you will need a "traditional modeler", either vertex or nurbs or both. A very powerful combo is Xfrog 4.2 plugin with a modeling host application such as Cinema4D, Max, or Maya. Then you can have the best of both worlds - and model practically every animal your mind can think of. Of course, it will take some learning time. Orio