Humans dislike inequity if they receive more than another individual and also if another individual receives more than themselves. Such a negative response to inequity has supported the evolution of cooperation by helping an individual to choose a fair partner for cooperative interaction (Brosnan, 2011). Recent studies have demonstrated that several nonprimate animals also show elaborate cooperative behaviors and are averse to inequity. However, very few studies have been conducted. In our study, we investigated whether domestic horses are sensitive to unfair situations and react to inequity negatively. The procedure was similar to that of Brosnan & de Waal (2003), with a modified task for horses. In the fair condition, both a partner and a subject touched a target to receive a pellet (a low-value food) when given a visual and auditory command by a human experimenter. In the unfair condition, a piece of carrot (a high-value food) was given to the partner by the experimenter (no exchange), and then the subject was given the command to touch the target for a pellet. In the expectancy control condition, the partner was absent and the subject witnessed a piece of carrot being shown in the location where the partner normally was, after which the subject was given the command to touch the target for a pellet. The subjectsâ reaction times were significantly longer in the unfair condition than the fair and control conditions. Moreover, horses also performed more indirect approaches to the target in the unfair condition compared to the fair and control conditions. These results suggest that horses were slower to perform the task and adopted a more indirect strategy when unfairly rewarded compared to the fair and control conditions. This is the first experimental evidence of inequity aversion in ungulates.