Regarding altitude changes affecting horses. My experience was this -
we took two horses from Seattle, Washington area to the Prineville,
Oregon ride last year. We arrived early Friday morning, the ride was
Saturday. We did the 50-miler. My horse did not pulse down as
quickly as usual. I couldn't figure out what was wrong with her. I
believe my friend's horse had similar pulse problems but not as bad,
her horse being younger perhaps. My only explanation was the change
in altitude. It's the only time this has happened to this mare in
two years, and it's the only Eastern Oregon ride we did in that time.
Not sure what the elevation difference was though.