I used psyllium products for horses since before they started making ones SPECIFICALLY for horses!(WAY back in '85, I bought my first couple of broodmares off a 'breeder' down on the TX-NM border near El Paso. One of them, in the first days after I got her home, pooped PURE grey sand...and I mean, PURE..not once but two or three times! I was HORRIFIED, called my then-vet..the only one at that time who 'did' equines out here...and she told me to give the mare some human Metamucil(this was BEFORE Sand-Clear or any other brand name for horses). Dosage was sort of 'by guess and by gosh', but it seemed to work...and that mare is still with me today, fat and sassy, at age 29+!)
I tried a number of 'brands', including the Walmart "Equate"...the no-sugar, orange-flavored kind. I did NOT moisten it with WATER in any way,though at times I mixed in a bit of corn oil just to 'stick' everything together; never in quite a few years of use had any problems(though would sometimes find a small, gooey 'blob'or two in their water buckets from what was 'stuck to' their teeth when they went to get a drink.) I even had to use it on very young foals a few times...the 'ground vaccumn' types; vet told me to mix it a bit at a time in a dose syringe w/ a little sugar-free pancake syrup. It was messy as heck, but also seemed to work.
I now feed on matting under their feeders, inside and out, mats swept off twice daily at least. Since I began using a stabilized ground flax, I no longer use a psyllium product, as a proper amount of the gr. flax, fed daily, is stated to be suitable to 'replace' what psyllium does, PLUS having several other significant benefits...as others have already pointed out. Seems to be working quite well. (I use Omega Fields...based on a recommendation by a vet on the CDL who also is a CDE driver.)
Margo