They choose their best horse that they thing will perform the best for the task on hand.
yes you can
you start to canter
Its ok if all the horses are not being ridden, but they will have to be ridden later.
I stay still or walking
you stay still or walking.
There have been many riders killed by horses.
most horses probably travel in style to the olympics: in airplanes made for them, or in horse trailers
17 years and 4 days
You start to canter in the indoor ring.
horse shows were inveted for riders to show their and their horses skills and acheive higher levels
There are horse events in the Olympics: Show-jumping But a so called "Horse Olympics" where horses participate in dressage,show-jumping, cross country etc are called gymkhanas.
Some horses may not be chosen by a rider because of : slowness, color, or size and ability. However, it may be because of how many horses and riders there are. For instance say there are 10 riders and 14 horses. 4 horses are left behind, probably the the runts of the horses, you know like I described above.