I was sitting in traffic behind a horse box the other day and I was wondering how did people first find out that horses can balance themselves in a moving vehicle? Humans need to hold on to something and even then tend to topple if the bus/tube brakes suddenly. Horses can even fly (in planes, not with their own wings obviously), and that can be quite challenging at take off and landing.
Also, do they need to get used to it from a young age so that they don't freak out? Are there some horses who will never willingly get in a horse box, and how would you work around that? Do they need sedation?