Mules are really good animals, problem is they only have one listing for them and that really is a bad view of them.
The "pony" as a mule would turn it into something like the general mules used on farms, or a common pack mule
Mining mule. It was shorter than the normal mules used on farms. Acutally deals with going underground very well, Have a huge pulling capacity (even greater that the farm mule), but did not run as fast and did not carry live weight well.
Racing mule. Some people thought it would be a good sport and started to breed for fast mules that could carry light loads. Historically one such mule was owned by a trackhand who would challange professionals in races for side money. His mule never lost to any thoroughbred.
Draft Mule. This was the king of mules when it came to size. Average was about 2 hands shorter than a large draft horse that was used as the parent. although shorter they often had even more pulling power than the draft horse parent. The best mix for this was the Mammoth Jack donkey and the Shire Horse. (Belgian horse would be the second best.)
Yes I spent several days on line when it was time to make mules for the game.