Stock cars were used for transporting live stock, such as cattle, mostly from the western states to the eastern states. These cars were used as early as the 1830s. They either had a single deck (the floor) or a double-deck, the latter would be used to transport smaller animals, such as sheep. Stock cars resemble box cars, but with ventilated sides and sometimes also ventilated ends. Trains made up out of predominantly stock cars would have a "drovers" caboose, which housed both the railroad's conductor as well as (non-railroad) people dedicated to the care-taking of the animals. Today, livestock is moved by road trucks and no longer by rail, with the last stock cars disappearing in the 1980s (the circus still used them until 2017). Note that S-scale poultry cars are listed under the "One-of-a-Kind Car" sub-category.

Below are links to the various reports that show all of the S-scale stock cars ever produced. Click whichever report gets you to the information that wish to find.
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These definitions, in alphabetical order, may clarify some of the terms used in the reports linked to above:
Don Thompson reported that S-Helper Service produced a total of 14,320 individual stock car models.
A lot of the S-scale models are those that have external bracing. Some angled bracing "leans" toward the center of the car and some toward the ends of the car. Some railroads had standards for those, and some did not or they switched from one design to the other as newer cars were being built. The Pratt Truss design had bracing pointing toward the ends of the car. The Howe Truss design had bracing pointing toward the middle of the car. Bob Hogan reported that the Southern Pacific was one of the railroads that switched design types over time.
The February 1992 3/16 'S'cale Railroading magazine issue has extensive research into the D&RGW narrow-gauge stock cars.