Horses love beer. Also, thanks to their body mass and the way their gut works, they can't get drunk on it.
Even today in Ireland, half a bucket of stout is considered a healthy treat for a horse that's just put in a hard day's work (Beer, incidentally, is actually a surprisingly good thirst quencher). The famous racemare Zenyatta apparently agrees. She likes a pint of Guinness. And only Guinness - she won't drink anything else. Beer is also anecdotally good for horses that suffer from anhydrosis or other overheating issues.
In some cases the beer is mixed into a bran mash.
The one thing you do have to watch for - horses really, really like beer. Given half a chance, trust me, they'll really, really like your beer. Some horses even work out how to drink it from the bottle.
I wonder if they let the Budweiser Clydesdales sample the merchandise...
(I'll take the carrot, but got any cold ones?)