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When referring to a person, a horse is an individual with adequate skills but perhaps lacking in curiosity, ambition, or opportunity. Often used disparagingly, for example "Joe from operations is a real horse. He'll never care about making his job easier."
Problems with the Term
No one wants to be thought of as a horse; it is demeaning and indicates a lack of confidence in one's ability to learn. Additionally, for someone to be perceived to be a horse does not actually mean that they are unambitious or incurious; they may simply be so overburdened by their tasks that all of their energy goes to simply keeping up with their daily work. A lot of the DevOps culture came from a place of wanting to reduce firefighting and stress so that we can have time to improve ourselves and our delivery of value to the business.