I use Amapi to model complex shapes. It's cheap and in theory you can make anything you want with it. You should try out a demo before you buy it though, some people love Amapi (me being one of them) and others consider it useless and un-intuitive. If you have a little more money you could go with C4D. The newest version (R8) has a modular approach, so you can buy the base module relatively cheap and do some nice creature modelling in it. Then if you want to animate the creature at a more professional level you can buy animation modules. If you want to go staight to the professional software you can use 3DSMax wich has some great modelling tools. If you combine it with Character studio it's powerfull enough to make almost any character animation you could possibly think of. I've also been told Maya is very easy on character animation. If you only need a modeller C4D, Rhino or Amapi are great, if you also want to animate your creatures the additional C4D modules are a great option. There's also a less wellknown app called Hash Animation Master and it's supposed to be a great and easy to learn program to create animations. I don't think it's very good at modelling though because it's only modelling tools are deformation based.

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