This study explored whether or not horses could respond to stimuli using a concept based on relative size. In experiment 1, after learning to respond to the larger of the two stimuli for six sets of two-dimensional (2D) training exemplars, one horse was tested for size transposition that used novel larger an smaller stimuli as well as three-dimensional (3D) objects (5 2D sets and 5 3D sets with large, medium, small, and tiny sizes). The horse correctly chose the larger of two stimuli regardless of novelty or dimension or combination. In Experiment 2, two additional horses were tested using a subset of the stimuli from Experiment 1. One horse was required to select the larger stimuli as in Experiment 1 and the other the smaller stimuli. After learning the task, both horses responded correctly to new stimuli and showed size transposition. These results suggest that at least some horses are capable of solving problems based on relative size concepts. Moreover, they are capable to generalize across situations that vary from flat, black shapes to objects of different materials and colors including balls, flowers pots, and PVC connectors. These findings support earlier research that showed that horses could categorize certain stimuli, and provide new evidence that they are capable of using some form of concept for problem solving. Understanding that horses have more advanced learning abilities than was previously believed should help improve training methods and management.