Grass. I believe oats and seed is fairly recent development. Historically, you had animals grazing by rotating pastures, or consuming dried grass (hay) when you couldn’t graze (eg winter).

Also interesting tidbit — apparently horses of modern sizes cannot eat grass fast enough to sustain their size. They require the higher calorie density of oats and such. Horses that get back into the wild apparently quickly revert to much smaller sizes with a couple generations (according to acoup.net, which I never verified further)

That assumes you have a lot of free pasture lying around. Odds are, the pasture is already in use, likely for sheep or cows.

All that's really necessary for the draft horse to wind up eating oats is that its economic contribution exceeds the value of the oats.

Yes, that’s why, as I said, no peasants owned draft horses before modern times, and very few land-cultivating ones owned any large animals at all.

I have the vague idea that in medieval Europe there would usually be one draft team which farmers would rent from whoever owned it. That supports the idea that owning draft animals was rare. But it doesn't so much support the idea that you wouldn't feed human-edible food to them.

Note that I said “human edible vegetables”, not “human edible food”, in context of a historical absurdity of a peasant feeding carrots to his horse. It’s as absurd as if 500 years from now, people on future equivalent of Hacker News claimed that in early 21st century, the poor complained about rising alcohol prices that they used to run their private jets on.

Now, I’m happy that your knowledge already allows you to infer that regular peasants did not, in fact, own draft animals, and I’m sad that it does not allow you to infer that the draft animals were not fed human edible food. Alas, that was the case: draft animals, which were universally oxen, would eat pasture grass, cut hay, and waste biomass like straw left over after growing grains. Human edible food was much too valuable to feed it to animals on a regular basis, and vegetables even more so, considering how little vegetables a typical peasant consumed himself (his diet was overwhelmingly grain based).