Do you think COB (rolled corn, oats, and barley) would work? I add it to the horses' feed in winter but not sure if the other grains would be OK for the chickens?
Normal chicken feed (crumbles or pellets) is a mixture that has lots of grain and some other things (protein, vitamins, minerals) to make it balanced. If they eat more of the normal feed to stay warm in the winter they are fine. If they eat a large amount of other grains, they eat less of the normal feed, and they end up short of protein and some of the other things that were not in the grains.
A small amount of COB or other grains is fine as a treat for chickens, but if you give them very much you will need to pay attention to properly balancing the nutrients of that and other food.
Since you already have horses: don't bother buying "scratch" for chickens. The COB or any whole grain will work fine for the purpose. The "purpose" of scratch is mostly to toss something and let the chickens have fun scratching and picking it up, but giving them very much of it will cause nutritional imbalance (see previous paragraphs), regardless of whether it was sold with a picture of a chicken or a horse on the package.
If you just want to give them a treat, without having to think about the nutrients involved, a general rule of thumb is to limit treats for standard sized chickens to about 1 Tablespoon per chicken per day (this works for scratch or whole grains or various other things, in pretty much any combination.)