I seriously doubt you'll need a passport, you are staying within the border of the United States. The only things required by law to transport a horse across state lines is an UTD negative coggins, and of course, UTD tabs on your trailer.
Anyway, it's more of a formality... I think most law enforcement prefer to avoid pulling over massive trailers with half ton animals in them on busy interstates unless they REALLY have to. lol
I bet it'll go fine.
We drink to our youth, to the days come and gone; for the age of oppression is now nearly done.