I would suggest doing a search on Google, so as to find and study ALL the info on ALL of the kinds of protective horse boots you can find! Most all are pretty pricey; honestly, I don't think anything that costs only $15 apiece is going to be what you want, as I can't imagine it holding up to any serious use. Measuring your horse's properly-trimmed feet, accurately, will be VERY important, for starters.
I have a pair of pony-sized Easy Boots, and just ordered another pair, so my 38" B mare will have them for all four feet. Some design 'tweaks' have been made on Easy Boots, and I do like them better than the older pair I have, for my smaller minis(which DO work OK, after I did some work on them.)The sizing of the Pony sized Easy Boots is EXACTLY right for my B mare, which helps a LOT; it is true, though, that Easy Boots *can* be a bit of a challenge to put on(removal is pretty easy)-but both actions require "tools"-a flat strap,(which now comes with each one-a good idea!) for putting on, and a flat-blade screwdriver, for removal(I carry the screwdriver in my spares bag, while driving, and for my 'big' riding mare, for whom I also have a set of 4 Easy Boots, I carry it in my saddle or cantle bag.)Easy Boots do not come above the hair line; although I like the look of the Old Macs, and of some of the others, personally, I have concerns about 'rubbing' issues with those that come up around the pastern, because as the horse moves, such boots 'have to' move back and forth over that surface....? IMO,the most promising type of boot I have found a website for, which looked as if it could fit minis as well as anything made, was an outfit in Arizona-can't recall the name of the maker just now, but they have both a completely CUSTOM made, and a 'semi-custom' type, in a complete range of sizes-AND, theirs can be redone-the bottom resurfaced-so the useful life can be extended! They are fairly pricey, though, as I recall. Good luck; it is a real challenge to find something that will FIT, STAY ON, AND BE COMFORTABLE FOR, minis!
Edited to add: Be sure to read all the info about any 'boot'; some of the less expensive ones are actually "barrier boots", made really only for medication, NOT for actually 'travelling', and wouldn't hold up to 'going-down-the-road' use.