It's the knee of the horse. It's up high on his back legs, close to his trunk/flank area. The next joint up in the leg above the hock (that backwards "knee" on the horse-which would be his ankle if he were you) and below the butt.
The horses "knees' on his front legs are comparative to your wrists.
Is it a gaited horse? I know some gaited breeds are prone to stifle lock. Where the stifle joint will pop or lock up.
The stifle joint is located where that little divit is on the front of the back leg, just below the hip at the top of the leg.