I suspect--or hope, at least--that they've built meshes that can be combined in a myriad of ways, so by combining the hundreds or thousands of combinations you can get millions of animal or plant types.
These are animated characters. So not only do you need to have the 'flesh' made procedurally (meshes/polygons), but also the 'bones' onto which the meshes are attached for animation.
Procedurally creating static meshes is one thing, doing the same for bones and animating it with a mesh so that it looks credible is a whole other story ...
I suspect--or hope, at least--that they've built meshes that can be combined in a myriad of ways, so by combining the hundreds or thousands of combinations you can get millions of animal or plant types.