One of the most confusing things about horses is that they stand on their toes - so their leg joints are in funky places compared to ours.
A human knee is the main joint of the leg...but a horse's knee is found on the front leg.
Our Fjord mare is stepping backwards. As you can see her right front leg is reaching back. The knee joint is the one that's flexed. It's called the knee because it bends the same way as the human knee, but it's not.
It's actually her wrist. The joint that looks like the wrist, the one above the hoof - called the fetlock - is part of her foot...that entire bottom of the leg is her foot and the "foot" is a toe. Her elbow is right at the top of the leg, tucked up against her belly.