I am looking at purchasing my first horse. The current owners allowed me to bring her out to my property to take her out for a ride and to see if we match up. She loaded just fine, but was nervous once inside. She stomped around for the whole ride (about 20 minutes) and was soaking wet when we got her home. Once we arrived, she back out of the trailer just fine. She acted a little worked up, so we walked her around for about 15 minutes to cool her down and calm her. I talked to the owners who said she did this the first time they trailered her, but said she got better each time. Is this something that can get better? We all assume she's had a bad trailer ride experience in the past.
She will get used to it. When training young horses to ride in the trailer, That first one where the trailer actually moves down the road will bring them back all sweaty and they stomp all the time they are in it. She probably just hasn't had many trailer rides, or its been awhile.
The more they ride the more used to it they will become.
I agree with Barbarosa. If your horse hasn't been in a trailer for a while it is not uncommon and should pass if you trailer often enough.
Especially due to this experience and a newness to everything else, I would wait a week if possible before riding her. I do that as a matter of course with every new horse I bring in. It gives them a chance to acclimate to their new surrounding and with me in particular.
I used to have a horse that would really have anxiety during a transport. It always helps to travel with a companion, however, that is not always possible.
One little trick I came upon is to put a "Lik It" toy in the van. That horse liked the apple version so much that she spent most of her time focused on that rather than the fact that we were on a journey.