> A new study reveals that horse personality is not only a matter of genetics, age, or breed but also systematically linked to how horses are housed, ridden, and cared for, and to the quality of the relationship they share with their owners.

I expect that genetics, age, and/or breed affect human expectations of the horse (e.g., 'this breed is aggressive'), which affects how they treat the horse, which affects personality.

Same with dogs. Pit bulls encounter lots of fear and human aggression, and are used for aggressive purposes by humans who expect that.

Same with humans, sadly.