I did try a garlic bug product on my big horses several years ago, and it did seem to help with the biting flies, don't recall if it had much effect on the mosquitoes; it did nothing to get rid of the filth flies (those that hang around the manure, but don't bite, they are just a nuisance). You need to start it before bug season, so it can get into the system; it may or may not work in time to start it now, it takes a week or more for the garlic to permeate the whole body. And, some horses won't touch the stuff, its so strong smelling and tasting. Also, I've read that too much garlic can cause anemia; but I don't know what constitutes too much. I didn't keep up with it as it got too expensive to too hard to keep them eating it; the saddle horses weren't too bad, as they got little grain, so they'd gobble anything they got, but the minis have gotten more feed, so are pickier about mix--ins.
