Ok guys:
EVERYONE LISTEN UP!
Well, that would be particularly bothersome. We don't know how to give custom player models different faces or decals. We can do colors, but that's in. Then, there's also trying to remake the animations for the centaur.
I don't know much about making add-ons, but I do know what a model is, and sort of what a script is. I don't know if you can copy and paste scripts, but if we took most of the player no-jet script (It's default) except took the script from the default horse of how fast it ran, and put that in the player no-jet script (removing how fast the no jet player can run, and replacing it with how fast the default horse can run) Then, we take the model of the standard player, remove the legs and feet, and take a default horse, remove the head, put them together, we got our model! And we could take the animations from the advanced horse.
Now, this is all my theory, I have no experience with making add-ons. Someone who does, please correct me, but I hope this helps.
Advanced Horse Download:
http://forum.returntoblockland.com/dlm/viewFile.php?id=2929 (Sorry I could only find RTB download link)