Horse developed sudden aggression towards another horse on the trail

Ok guys, I have a 6 year old 16.1 spotted draft gelding! Such a cool dude! Always been absolutely perfect on the trails and never taken a wrong step! I trail ride frequently and often for 4+hours. Maybe 3-4 times a week on a longer ride! Yesterday halfway through our ride, about 2 hours my gelding decided he wanted to full on try to kill another mare one of my girlfriends was riding! Full on charging, ears pinned at her, then was bucking and rearing! I was able to ride that out and correct him but the whole rest of the ride that was 2 more hours he had a full on vengeance against her! He’s never acted like that nor even bucked or reared before! I unsaddled him, check his feet for pain, his back, the saddle tried to recreate pain, though maybe a snake bit him and he took it out on her but I came up empty! After we got back we put that said mare away and he was perfect! I’m at a loss as to what the heck happened! Has this happened to anyone before!?

What was the mare doing to him?

Absolutely nothing… minding her business

He might just hate her. We heard banging at the barn this week before daylight. I go down and one mare has my filly penned up in the run in shed. They will pin ears at each other across the aisle when stalled. Have never liked each other.

Hmmm. Not passively aggressively refusing to move out of his space?

Have seen this once before, horse VS mule. No matter what we did horse was determined mule didn’t belong. Horse was also 24 years old and docile. Never did figure that one out.

You might not be able to see this, but horses have very subtle ways of communicating with each other. The mare may not have liked your horse and he sensed it and felt threatened. He threatened her right back.

I agree with this. It happens with dogs, too.