Well there are over 6000 members in the AERC (horse endurance racing organization) and many of us ride with HRM's and GPS's. Currently there is none, as in zero, software that can be used to estimate the fitness of a horse or show the improvement in fitness when run multiple times over an endurance training loop. There are similar large groups in the UK and Australia so you might be first into the gold mine which could be a good thing in this economy.
I realize some of the output numbers for a horse (like watts) might not be really correct (you add a disclaimer) but the "change" in watts between runs on the same training loop would be correct and would be a great indicator of a "change" in the fitness of the horse. Right now horse riders just have data. Gain, Distance, Time and HR and quite frankly looking at raw data tells me nothing about fitness.
I vote you whip up some code and let a few of us horse types test it out then park a dump truck under your PayPal account to collect the money.
Thanks for your help,
Fasterhorses