Any chance a careless poacher was involved jimmy? You periodically hear of the odd black bear and a few wolves around these parts but not often. Coyotes attacking horses? I tend to agree with those that suggest "if the horse was weak or sick perhaps?" I know the local shepherds in this area all keep a few donkeys to "care" for the flock and they do a terrific job of keeping the coyotes at bay. (and "any" other predator).
We have some large flocks in the area and you "never" see sheep and lambs without a donkey escort.
Subsequently, . .I would think a "healthy" horse could do what a donkey does? Good luck with your investigation,...too bad the evidence seems to have disappeared. It makes me angry when people allow their animals to get "outta-sight" for extended periods like that and don't seem to "miss" them!
Coyotes attacking humans? The case in Eastern Canada yes, but not often otherwise. A couple years ago in the summer of /07, I was working out in the back field around mid-night with my cab tractor and had full size coyotes bothering me. I threw on all the lights, flashers etc and opened the throttle and tried to scare them off chasing and banging the bucket etc. Didn't help much.
Had a flat front tire shortly after and as I was not armed at the time, (often have "something" long or short with me),..was actually afraid to get out. Yes I opened the door quickly and yelled which caused them to run a few feet and come right back,...so I ended up skidding the front wheels home on the bucket, steering with the brakes.
Went back out on foot, a little better armed ....not a coyote in sight. Perhaps it was Deere-John: the Big-Green-Noisy, "Growling-Monster" with the Many-Bright EYES that had attracted them in the first place? BUT they had me worried at the time!
The best to you,
. . tug