The stallions, like all stallions, are very jealous and possessive about the mares, and the lead mares are protective of the herd. They are incredibly fast and aggressive in captivity. I expect at least in part it's because even if you give them 100 acres, you are still dealing with an animal that evolved to have virtually unlimited grassy steppes.
Joan Embry was almost killed buy a przewalski's stallion who reached into a moving ATV and pulled her out the back.
I am by no means being critical of keeping wild or tame equines, but keeping even the domestic ones is challenging (mine are free on 4500 acres of rough terrain and grassland, and come in when they are called, which is as close as I can manage to steppes
These were gone in the blink of an eye, but there is going to be a batch of Grab bag unicorns going into the online store this week ( January 2013) sometime, if you are interested in those...they are bright colors