I don't think the horse riders were being malicious as much as they were curious. I know if I saw a new trail while riding a MTB, that didn't say that MTBs weren't allowed, I would ride it to check it out (Coralville?). Curiousity and exploration were the culprits. Just like my kids violate the unwritten rules of the paved trails until I explain them.
Granted we need to be vigilant of horse riders in this area. Squaw Creek (horse and MTB) riders need to learn the rules of the shared trails and how to get along. Many of us have never ridden around horses before and many horse riders haven't ridden around MTBs either. It works though. There are lots of shared use trails in this country.
In my opinion the rider of the horse is in a more precarious position. Spooking the horse could throw the rider and cause serious injury to both horse and rider and MTB rider. Stop, yield to the horses when on the shared trail. Do not endanger your or the horse rider's life by being inconsiderate. I'm sure our trails will be short lived if we cause problems with the horses. No matter who is at fault, we are the new guy and those problems didn't happen until we got there.
Temporary signs have been posted. Noel is coordinating the permanent signs. Lessons have been learned. Let's get on with some riding.