Oh you probaly could if you realy rode them hard and they were out of shape.Heck you could kill Montana horse if you are riding them hard and they are out of shape,seen that happen about 8 years ago.The problem when that happens is you have to pack your horse out cause of the bears,plus you lost a horse.Some guys afew years ago abandond a dead horse in the Bob and Forest Service blew it up with explosives and the bears were eating horse all over the place.Bad deal cause then the bears get the taste for horse flesh.
Heck by the time you get here the horses should be fine,if your worried alow a day to ride them for a few hours and get them used to the alltitude.
BBJ