Are they polled? Is your fencing barbed wire?
In my experience horse fencing just doesn’t cut it when the cows get hungry/horny. And for obvious reasons, I wouldn’t put a horse in barbed wire.
We have a very “nice” bull. He has never been pushy or rude, but he is 2600 pounds of do-whatever-I-want, including climbing over or through fences.
We have also had several horses poked by horns through fences because they were messing with the cows over the fence.
My 'pet’s cow is not with any of my horses because she is a miniature and $$$ and I do not want to risk her getting kicked or chased. She does love a big TB on the farm, but the risk is too great to let them frolic together. Of course, she is halter broke and dehorned, but we will watch her closely when she has her first calf this year.