Whenever I have had a horse I can either give an oil to or that is already on some form of oil e.g. vegetable oil, I have popped a few garlic gloves in the bottom of the bottle.....making garlic oil! You need to do it in small bottles though, otherwise you will end up with rotting garlic in the bottom of your oil! very cheap way to get garlic into your horse...the oil does make some horses fizzy though!!
If you live near a chinese hypermarket - like Wing Yip in Croydon - their garlic powder or granules for human consumption are about 1/3 to 1/2 the price of equine garlic. I think Wing Yip does it on line too.
I did think twice about using a supplement not designed for horses but garlic is garlic. I prefer the granules to the powder simply because the powder disperses more quickly and I tend to end up with the feed room stinking of garlic! It is cheaper than granules though and might be more acceptable to a fussy horse.