Well, it kinda depends on how the trailer is set up. Is it a slant load or more of a stock type trailer? I think (not sure) that most trailers do have a recommended weight limit that can be safely maintained on 4 tires; anything above that needs to have 6 tires. I am not sure that I would be comfortable with stuffing an extra horse into a slant or straight load type trailer that is specifically designed for only 6 horses. However in a stock type trailer, for me, it would really depend on how big the other horses are, how comfortable they are with being a bit squished, and whether we could stop mid-way to get them out and give them a while to walk around, and what kind of condition the trailer and tires are in. Basically, I am not sure that I would want to risk it. It wouldn't take much for a horse to get down in the trailer and things to go very bad from there where they get trampled and kicked with no room to get away.