We once brought a roping horse from Oklahoma to our place at 7400 ft.
Granted, he was 16 yrs. old and only used to roping (burst of speed,
turn to the left) but when we took him out on short trail rides he
huffed and puffed. I think altitude MIGHT affect horses, like poeple,
because he eventually acclimated, even though he still mostly did burst
of speed, turn left. There's less oxygen up here. And probably more
DIMR!
Bambi