In this tutorial series I tackle illustrating, rigging and animating a cat in After Effects.
The first thing people should know about rigging a cat versus rigging a human is that it's double the work (because it is double the legs). Also animal walk cycles are all just slightly different. Dogs vary from cats, cats vary from horses, and so on. So each animal rig is it's own unique beast so to speak. This is why it's always good to have lots of references. I have several books that reference various animal run cycles with photos and information. These are vital to understanding just how different animals walk, because if you don't understand the basics then there is no way you'll be able to deviate and add your own creative spin on things.
Another thing to note is that most animals require an extra bone by the shoulder or foot, so this adds to the complexity of your rig.
Finally, the other difficulty with animal rigs is that they often require extreme flexibility, especially in cats. If you've ever seen a cat run you'll notice that their torso often stretches very wide on the jump and compresses very small on the landing. This doesn't seem like a big deal but in terms of looking realistic, getting this to work is what will set your rig apart from others. This flexibility can often be hard to pull off without breaking the rig as well.
In the following videos you'll see how I tackle these various challenges through the creative process. I found this to be one of the most flexible cat rigs I've ever created and I had a lot of fun animating with it.