As much as I hate to take away sales from DAZ, I would be negligent in pointing out that there are several freebie Poser centaurs floating around the 'Net, based on the P4 human and horse figures.

Nerd 3D: female centaur (Posette)
Cloudland Castle: male centaur (P4 Male)

I haven't tried either, myself, so cannot vouch for how well they work. I also don't know if they've each been combined into a single figure, or are just the two figures stuck together.