In MOST areas, its 1 acre per animal. To be absolutely sure, I would consult your town/city zoning board to see what the actual requirements for your area is. This is the only way you will know for sure.
I’ve never heard this one before. If there is a legal amount of land, I guarantee no one goes by it where I live. I see horses in little round pens, and in peoples yards that are not even close to an acre.
Last year I aquired a starving horse that had been kept in a small rented stall for quite sometime. I think it would be a great law if you had to have at least an acre to own a horse.
I’ll keep watching your question to see if you find any answers.