Almost no research has been done on designing pinball games although much research has been done on designing video games. We are interested in designing pinball games on pinball machines to teach computer science students about how to interface to mechanical systems in a fun and motivating way. Thus, we have developed a pinball machine interface between a PC and a recent Lord of the Rings pinball machine. We demonstrate that it is easy to innovate pinball games by designing and implementing Pinhorse, a pinball game that avoids some of the design problems of existing pinball games. For example, it features a true multi-player mode where each player directly influences the game of the other player within a limited amount of play time. This paper describes both our innovative pinball game and the hardware and software of our pinball machine interface that enables game designers to develop such pinball games on real pinball machines.