horse gaping mouth - reason

Question

horse gaping mouth   hello, i am rohit from india. i am totally new to horse training. i have attached an image about a horse being ridden by an exercise rider. may i know why this horse has opened its mouth while working at  moderate pace with the rider. is this horse evading the bit or unable the bear the pressure in the mouth due to worse hands of the rider?

Answer
This could be a little of both.  Horse may be wanting to go faster than rider is allowing it to go,as the rider, in cases of racehorses, have been told by the trainer how fast to pace the horse, and thus fighting the bit.

Could also be having bit of a temper tantrum and has mouth gaping because of that too, showing his disapproval and displeasure.

Could also be tiring and pulling more air in through mouth.

And rider could be attempting to slow horse down, and horse doesn't want to do so.

Some horses do this as a habit too, whether because they have gotten used by a heavy handed rider, and do it no matter who rides.

Bit, teeth, or parts of mouth could be bothering horse too.