Are male horses more domminant than females?

As a general rule, most mares get moody when they come in heat. Stallions are also generally as less well-behaved than mares in estrus, mostly because of the fact that their hormones start raging when they smell a female in heat. Geldings on the other hand, are generally the best tempered type of horse out there because they have been castrated and do not have the raging hormone issue going on like mares and stallions do.

However, every horse is different. Some mares will be better than some stallions, some stallions will be better than geldings, and some mares will be better than some geldings. It all depends on how they've been socialized and trained that really depicts how well-tempered they can be.