This link might help you, or mehbee not. Generally TheHorse.com gives some pretty decent information but this one seems to be more of a "yes, I guess not" Pre- and Probiotics for Horses | TheHorse.com
There are PREbiotics, PRObiotics, and pre-probiotics. Within the above link (to the right of the page) there is another link where you can download an overview of Pre and Pro biotics that hopefully will help.
To feed or not to feed them (and which one or both) depends on the horse's digestive health. There reaches a point where the horse's age also plays a huge part.
I have my 25+ TWH with EMS and hindgut ulcers is on the PREbiotic "Succeed". Succeed and being fed three times a day has stopped him from colicking.
Two other horses are on Daily Start (by Equi-Shine) which is both a PREbiotic and a PRObiotic.
The 27 yo Arab was a starving horse poster child when I rescued him 20 years ago. He has always had a finicky digestive system. While his manure has never been runny, he has always had a "wet" bum, for lack of a better word. He deals with occasional gastric stomach ulcer flare-ups.
The Spring grass should really be making his bum wet but it's as dry as it can be and has been that way since I put him on the Daily Start.
My 19 yo TWH has digestive issues that has the vet stymied. I put this horse on Daily Start, as well. Whatever "those" digestive issues are, the Daily Start has markedly improved them. They're still there to some degree (exhibited by loud and somewhat excessive gas) but he is nothing like he was six months ago, Before Daily Start.
Meaning yes, pre and probiotices have a place in a horse's life. For my three it's a permanent place.
For some horses, it can be a temporary place after the horse has been wormed, has come off oral pain killers like Bute, aspirin, etc. or after anything that stands a chance of disrupting that particular horse's digestive system.
Also, Ogliosaccarides are PREbiotics; anytime you see them listed the cost goes up. I'm not chemically smart and don't understand why but I've noticed the cost of PRObiotics can triple when the PREbiotic has been added in.
Never buy a Pre or Probiotic that doesn't list ingredients and doesn't list CFU's, which need to be in the Billions (with a "B") to be effective.
Hope that is of some help