I think you mean Sports Medicine Boots. One of the most popular ones are the SMB Sports Medicine boots
SMB Sports Medicine Boots
They are commonly used for barrel racing, but people use them for flat work, trail riding, and a some for jumping. One of the ideas behind them is that they help support the fetlock so it doesn't hyper-extend. I personally am not the biggest fan of them, the fetlock is meant to go down and absorb shock, and if a horse is landing with that much force I really don't think a boot can stop that.