Thanks everyone, yes I just did a bit more reading on it. Im just paranoid to use anything new with the very sad threads on here about horses being poisoned by things I never even knew were toxic to them.
So according to my reading, yes, it does make the horse manure useless for gardening as it kills a lot of beneficial bugs. But Im not actually planning to feed it to the horse at all, I just want to sprinkle it around the house to kill these evil fleas. And yes, it looks like there are gardening grades (to kill pests) and food grade (for treating stock for worms). Obviously I'll have to get food grade as he's bound to at least try it.
And then Im worried because its a powder and you're supposed to feed it moistened in case of choke. Well, I'll just try a tiny sprinkle first maybe....
OR if anyone knows any other non-toxic ways to get fleas out of soil that wont poison a horse??? Im open to all suggestions!