Long haul trailering

[Bob & Amber Roberts <mlaboure@ntws.net>]

One of my boarders plans to haul his horse on a 12 hour trip (split into
two 6 hour days).  He has found that this horse is more anxious traveling
in his somewhat closed up 4 Star aluminum slantload than my more open
straight load 4 horse combo.  He can't figure out if the horse is nervous
because it's a long trip or because he's alone in the trailer.  He asked me
if I'd check out some form of mild tranquilizer to give the horse before
hauling him in his trailer again.  He is an extremely cautious driver by
the way. 

Thanks for any clues.

Amber