No, just no. Most horses are not fed grains of any type. They eat grass and hay. Grains aren't really great for them anyways. Performance horses and elderly horses will get supplemental style grains, but a horse needs to basically eat 24/7 to keep their guts properly working.
The human brain requires a relatively huge percentage of energy to work. Most plants aren't calorie dense enough to get the job done; vegetarianism is a modern luxury.
Veganism is a luxury afforded to us who have access to vast amounts of fresh produce all year.
Even 100 years ago I don’t think being vegan would have been possible year round, as it would be nearly impossible to get the variety of produce needed to sustain a person in the winter months.
Look guys, not looking for a fight here, but maybe just don't tell the people who are already in somewhat of a war with vegans to go and eat literal grass 😅