I know of a few that were inherently dangerous to be around, for no particular reason. Well handled as youngsters and in later life, but not the sort of horses that you could take your eye off of when around them. Several were sired by the same stallion, which was a horse I would not have bred a mare to for that reason (among other reasons). One of these was bred by some good friends of mine, and she was a horse you did not want to be around or near. My DH was walking through her paddock one day, at her owner’s house with the owner, and the mare attacked, at the run, across the paddock, trying to kill both the DH and her owner with teeth and hooves. They only just managed to escape the attack. She had a lackluster race career, then my friends wanted to breed her to my TB stallion. I refused to breed the mare, and suggested that they NOT do that. They gave the mare away to another breeder, who did get the mare bred to his stallion… I think it was pasture bred. I don’t know how the stallion survived this. But, when the mare was due to foal, a foaling attendant was present, and the mare was being dangerous. He went into the stall, and grabbed the newborn foal, who he felt was in danger, and pulled the foal and himself into the corner of the stall. The mare lambasted the two of them with multiple kicks, killing the foal instantly, and severely injuring the foaling attendant, broken ribs and pelvis. That was the end of her career as a broodmare. The SPCA eventually took possession of the mare, and passed her on to someone else. Years later, my friends, who were her breeders of record, got a phone call from a girl in a remote town, who told them what a lovely horse it was, and how much they loved her. The child was taking her to local horse shows, winning prizes, and everything was lovely. The mare must have been 20 by then. I would have liked to see that personally to truly believe it.