There is plenty of research out there about functional electrical stimulation for humans, but (as usual) it’s been limited for horses.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4749012/
http://www.equinew.com/index.html
FES is now being offered by a local veterinarian. Who has personal experience with FES? What condition was being treated? What protocols did you follow (how many treatments, and at what intervals)?