The Prayer of the riders:

It is obligatory for a Muslim who performs prayer while riding his animal (or what takes the same ruling like a car…etc) to face the Qiblah if possible. If he cannot, then it is permissible for him to face any direction in prayer. He is obliged to do whatever he is able to do such as bowing, standing, prostration, tranquility, if he cannot do bowing and prostration, then he has to beckon with prostration while he is sitting and bowing while he is standing if he is able to stand. Whatever he cannot do, it is not obligatory for him to do it.

If one is able to dismount, or it can be combined with what follows it and one is able to dismount in the time of the prayer to which the other will be combined, then one has to wait until he dismounts and pray a complete prayer.