Horses make their own probiotic. It is called 'poop' (amonst other things).
Whenever I have a horse that is not doing well, has been recently dewormed, sick or on antibiotics, or has some other health or digestive problem, I just put a scoop of fresh manure in its pen or stall every day. I like a donor that is healthy and parasite free, preferably a slick, fat easy keeper that is eating a similar diet. That horse will have a super high number of the beneficial bacteria that produce the exact enzymes that a horse on a similar diet needs. One small fecal ball will have more good bacteria than a tube of commercial probiotic.
This is nature's probiotic. A foal will nearly die if it does not have access to fresh manure. They start eating their mother's manure when they are only 2 or 3 days old. If a horse needs it, they will eat it.