Yes as Verona stated - a welder could make some dividers for a stock trailer, if required. You may find some of the newer stock trailers actually do have a divider or two in them as standard equipment.
With regard to that trailer, it does look similar to the trailer Roy Rogers used in the movie "Under California Skies" in 1940 something. That particular one looks small and may be a tight fit for anything over 14.2 hands; it also looks like it's got some age on it which means a very careful structural inspection should be made; and the open roof concept is not a safe way to go as it doesn't protect the horse from the elements, flying debris, etc. (and I don't even want to think what would happen to horses if there was some sort of accident).