I recently purchased a 4-year-old gelding. He is extremely aggressive toward my other gelding in the pasture, and I’m unable to turn them out together. I am able to turn the horse out with my pony mare and pony gelding, but he “herds” them constantly and often will unsheath his penis and sustain an erection around mares. When left stalled, he exhibits other strange behaviors such as constant stall weaving and defecating in his water bucket. Please advise.                 Martha, Tennessee

Let me start by saying that I have never figured out defecating in water buckets. It happens; I’ve known several horses which did it, both mares and stallions. In a male, you could imagine it is some sort of marking behavior, since stallions in natural social groups at liberty make stud piles near water holes. Maybe some of our readers have some theories or tested explanations. In your gelding, the defecating in the water buckets might or might not be related to the other problematic behaviors.

The other behaviors you describe–the herding of your gelding and mare, the aggression toward the other gelding, and the sexual arousal around mares–all are probably related. They represent stallion-like behavior. The weaving in the stall might represent his thwarted stallion-like motivation to be out controlling and protecting his herd. Or like the defecation in the bucket, it might be coincidental. If he seems eager to get out of the stall (calling out to the herd, hyperactive) the weaving likely is related to being left behind from his herd that he urgently wants to protect and control, as a harem stallion would.

So why does your gelding act like a stallion? Occasionally, in evaluating geldings with behavior such as you describe, we find that they are not completely castrated. That means they might have some r