I guess it depends what you mean by good cow horse. Most horses I've seen around here have been made into decent ranch horses. We have a lot of QH, but I've seen Arabians, TB, etc... that were great ranch horses, especially on big ranches where they needed to cover a lot of miles. People in our cattle sorting club use everything from QHs and Paints to Appys, Arabians, Standardbreds, and Morgans. My one horse is a Percheron/QH cross. The same horses are usually used on cattle operations
In fact, one of my relatives, who I no longer speak to unfortunately, bred Arabians and TBs for ranch work, and they were fantastic.
But occasionally you'll get a horse that is terrified of cattle, or just wants to run, or is hopelessly lazy, or really clumsy, etc...
As one of the other posters said, get your horse responding to your cues, then introduce him to cattle. He might not be totally comfortable with the cattle at first, but he should settle in and start connecting your cues to working cattle.