I think they are knee boots, just not the kind designed for riding, they are more for protecting injured joints, holding dressings on, 'sweating down' knees and possibly travelling. No I would not ride in them as they are designed to be put on securely, tight to the knee unlike the knee boots Jess describes which have looser fastenings so allow the knee to move. These ones wouldn't stay in place for riding and would make your horses knees very hot so would just be annoying and pointless. The person selling them is probably only doing so because they bought them thinking that as they were knee boots they must be able to ride in them but soon realised they were actually no good for riding and they had no use for them.