Medicine boots (also known as soaking boots or poultice boots) are hoof-shaped boots that are worn over a bio-horse's hoof. They're typically made of PVC/vinyl and are used to either protect the hoof, keeping it dry and clean, or alternatively to contain liquids and allow the horse's hoof to be soaked (typically with a medicinal liquid to e.g. treat abscesses) or contain a poultice.
While they could certainly be used similarly in pony play (like in some sort of vet or farrier scene, for example), they're most common use in pony play is as hooves. This is because they are hoof shaped, easy to use, and inexpensive. Moreover, they can work just as well as other hoof mitts designed with human bondage in mind because the hard PVC prevents the wearer from using his fingers, and if sized well, the Velcro strap with easily hold the boot in over the wearer's hand.
They can be worn as either hand hooves or hoof boots. The former is quite easy, just have your pony make a fist, stick his hand inside the soaking boot, then tighten the Velcro around his wrist and voila! no more pesky fingers, just hooves! You can also add polos for additional security in keeping the hoof on your pony's hand. If you want even more security, you can tape up your pony's hands before putting them in the boots, then he'll really be helpless.
The most important thing is to get an appropriate size boot. If they're too big, your pony will be able to slip them off even with polos. I would suggest trying them on - discretely of course - in a tack store. If that's not an option for you, then consider using a horse hoof size conversion chart to determine what size boot would best fit your hands and buy from somewhere that allows returns, like Amazon, so you can return the boot if it doesn’t fit.
It's equally easy to use the medicine boots as hoof boots, just have your pony stick his foot inside one and tighten the strap. However, because soaking boots don't have support for the heel (after all they're meant for a horse), and due to the flexibility of the material, they will bend easily if your pony puts much weight on his heels.
This isn't necessarily a bad thing. It can be a useful way for ponies to have hooves without being forced to walk on their toes, and the bent hoof doesn't look too bad - from the front you may not even notice it.
Nevertheless, if it's important to you that your pony walks on his toes when wearing hooves, there's a quick and dirty solution: use a metal rod taped along his foot, have him put his now forcibly extended foot in the boot, attach the Velcro and wrap up the polos to give a more streamlined and secure look. In less than 5 minutes, you've made your own pony point trainers that even look like hooves! However, I don't recommend doing this due to the risk of injury from falling and the metal itself. So do be very careful if you try this.
The most well known brand is probably the Davis brand medicine boot (blue and black boot), but there are a few others. Most are functionally similar if not the same. When adapting them for human pony play, I think it's merely a matter of aesthetics as to which one you pick.
Hoof soaking boots are a great way to get a pair of cheap, decent looking hooves. They require no work or effort (aside from picking the right size) for hand hooves and only minimal effort when used as boots. When worn as boots, the pony has the option to walk on his toes to keep the overall shape of a horse's leg and hoof, or, if he's tired, he can rest his heel while still keeping a decent hoof shape.
You can buy them from pretty much any medium to large tack store. If you don't have a tack store local to you where you can try them on, you can always buy them direct from Davis Manufacturing (who make the Davis brand), or from a major online retailer like Dover. You can find them on eBay as well (where you easily find other brands, which may have a color you prefer, etc.)