A horse of mine slipped on the road and went down on his knees. Fortunateky no damage but have always worn knee boots since... think mine are woof wear..
I've bought a couple of pairs off Ebay, I'd only buy leather ones, they are far more substancial & will protect the horse more in the event of a stumble.
At the moment have the all in one knee and brushing boots, but prefer the separate leather knee boots, as I think they slip less and offer more protection, though they take longer to put on, and can be tricky to keep clean, especially in the mud!
Have had to dismount mid ride to remove a branch from the all-in-one knee boots, as they have a big flappy area that things can get caught in. (Was very amusung for my friends to watch me re-mount from the floor an impatient 17hh+ who would not stand next to a perfectly good bank, so that I could get on!)
I used them as Ralph always ended on his nose after stumbling....cant remember the make. However, I discussed his stumbling with my farrier and he suggested natural balance shoes.......a year on, so far so good
I use them always on hacks just to be on the safe side. Make sure you get the type with a bottom strap as the ones with just a single strap can flip up if the horse stumbles so serve no purpose.