Horse goes sideways

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

My husband's Arabian gelding just turned 5 yesterday and he is
conditioning him to do his first 50.  After a training ride yesterday
(what a way to celebrate a birthday!), he mentioned to me that the horse
was tracking sideways.  This horse usually tracks straight as a arrow.
Horse was doing about an 8 mph trot with front end going straight ahead
but the rear end was off to the side.  He would try to straighten the
horse up but horse would then go straight on front and slant rear toward
other side!  He does not do this at a canter.  It was so bad that in an
area of trail where there were two ruts side by side, the horse's front
end was in one rut while the rear was in the other!  I don't know how
long this has been going on since it was the first I've heard of it.
Does anyone else have a problem like this or any ideas of what might be
causing it?  If so, how do you cure it?

Thanks.

Amber