A stock horse, by original definition, is a horse that is used to work stock...AKA, a cowhorse. They should look like the foundation quarter horses that have been posted above; compact, short-ish, broad, strong, and deep through the girth.
A modern day example of a "stock horse", IMHO, can be seen by looking at Aus stock horses. They are bred to have the endurance of a TB, the cow savvy of a good QH, and the build to do anything you want them to....and do it well.
Here in the US, you'll find most traditional looking stock horses on ranches and in the cutting/working cow horse/roping arena.
For example, the 6666 ranch here in Texas. They are one place that has what I would consider the definition of "stock horses", even though I don't like all their stallions and there are a few in there I would have gelded, you can't say that their horses have never done anything to prove themselves.
Performance Stallions | 6666 Ranch
However, most modern definitions describe a stock horse as nothing more than a non-gaited or non-exotic breed which includes QH, Paint, Appy, and TB.