It looked like a few there on horseback had six-shooters - not much good, but I did see pictures of one or two with more capable longer range weapons up behind the concrete bridge dividers behind them. (If that was at the same location).
My main concern is that the BLM/contractor goons had supposedly non-lethal CS grenade launchers, but to launch one in the midst of a group of riders would likely result in the trampling of some of the dismounted civilians. IOW, the potential use of these weapons by the government WAS lethal in some respects.
Those horses are very likely ranch horses used to working around cows, coyotes, wild horses, etc. It takes a lot to un nerve them, not saying it is impossible, but the horses would likely not spook and panic. They were used in violent warfare for thousands of years for a reason.
The line of protesters, including the guys on horseback, were just for show. The very real threat was from the guys holding the high ground with long guns. I've only seen pictures of the one on the bridge. I'm sure there were others that slipped away from the crowd and quietly took up positions.