This is a late reply to this question, since Erik actually came to us for this project.

We sell horse mocap data at http://www.kinetic-impulse.com/mocap_data/index.html.

We have all the walks, trots, canters, bucks and kicks, as well as all the transitions such as walk to trot and walk to stop.  We work with leading equine biomechanics research labs and professional feature film stuntmen so are also able to provide new custom data as required.

Best,
Richard

To update this old thread.... (Stuart contacted us through the website)

We have just released the next generation of our horse mocap data library at:
http://www.mocaphorse.com
http://www.horselocomotion.com

We also now have a fully skinned and rigged 3D horse model, poster images (like Eadweard Muybridge's "Horse In Motion"), image strips comparing the different gaits, movies that can be used for rotoscope backgrounds, and flip books for use in company  promotions and trade shows.

Best,
Richard