Q. I took my best trail and traveling horse, a 7-year-old Tennessee Walking Horse gelding (gelded at age five), on a trail ride recently. He has been on other trips of similar distance and length of stay and has always been great in groups, even very large ones. He is very tolerant of other horses around him. On this trip, my friend took her well-behaved Spotted Saddle Horse mare. The horses had met briefly in the past, but do not live/board together.

This was a gathering of approximately 50 to 75 horses, so all trail rides were large. On the first ride, my friend and her mare opted for the longer ride, and I took my less conditioned gelding on the shorter ride. As the "long ride" group separated from my group, both of our horses displayed separation anxiety by crow-hopping, jigging around, calling to each other–behaviors that were unusual for both horses individually. Once we got moving, my horse settled down and was the trail-savvy boy I am used to.

When in their accommodations (10×10-foot stalls–both are used to much larger areas), my boy became very aggressive toward other horses coming near "his mare" and bit a gelding to one side as well as the mare (no stitches required, but still extremely unsettling to all humans). He even kicked at me once, which he has never done to horse or human (he loves people to scratch, or horses to scratch or nibble, on his butt). That was immediately punished.

When we did ride together, he didn’t want to let the mare out of his sight, and would pin his ears and act aggressively toward other horses that came near "his" mare