One way is to put them in a box stall where they can move around comfortably and lay down if needed. And of course have access to water and hay if needed for them. Hope this helps!
It's a good question... I had a sick horse in a truck, and to be able to take him with us, we had to take him medicine and a certain diet (just the day before). In fact horses are sick in transport because they stress .. but stress in a horse can cause the cortisol rate to increase, to an increase in heart rate, to a behavior of a disoriented horse. Afterwards there are 4 basic rules essential to the well-being of the horse during a transport: the absence of hunger, thirst; The absence of fear and distress; Absence of physical and thermal stress; The absence of pain, injury and disease. If these 4 "rules" are respected, it means that the horse is very sick in transport ... In this case I advise you to call a veterinarian so that he can tell you what to do (this is what I did).
I hope this helps :)