I personally wouldn't, unless those horses were specifically raised for the meat industry. Horses that had a previous life in a barn, whatever their purposes may be, could have been exposed to drugs and pharms (e.g. bute that is quite commonly used) that are not safe for human consumption. My rule of thumb is, if I would not eat it, I do not feed it to my animals.
If it is a business that is supplying to zoos, rehab centers, etc, I would be a little hesitant because they look for cheap sources of meat in order to keep running costs low. You could ask for more information, of course, and trust that they are honest about it. I may be way off course here, but it is not implausible that the meat comes from auctioned horses who were - sadly - treated like commodity who no longer served a purpose for Man alive.
I am an avid rider, and couldn't consume horse meat but if you can purchase safe meat from a reputable source, it is a very healthy 'exotic' protein for your dog. I have also interned at a zoo as well as at a wildlife rehab center, and have little knowledge about the meat deliveries when I was a food prepper for zoo and center's residents. Hence, my advice to be sure where the meat comes from.
Let me know what comes of it. Good luck.