My gelding lived with horned Highland cattle for a couple of years both bullocks and heifers. Only small groups of 3-4 plus him in a 6 acre field.
At 14.2 he was the herd leader they would graze together and he would often lay down with them. The only problems I ever had were him chasing them off the hay in the snow and the cob gelding he lived with for a while chased a couple over the stone wall fence.
TBH I was very worried about the horns to begin with but when it came time to move him back home and to a livery yard I was more worried about turning him out with other horses again and him getting kicked!
Have horses kept 3 cows and no the cows back off, next dooor neighbour has highland cattle and they are as soft as they come only time they may give trouble is when they have calves.
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Friend's highland cow gored their pony very badly and the pony was never the same again. IMO I wouldn't risk it.
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That was very unfortunate, but in the same vein, two of my mares had a bit of a turf war, and the Vet bill for sewing them back together was over $700 for the pair.