I'm no horse anatomy expert but I'm pretty sure that's not how horse legs work, but eh that's Gold Ship for you I guess...
The pastern joint can't bend like that, yeah (what limited movement it is capable of is only for shock absorbance). But Golshi will find a way...
On a side note, it's the equine proximal interphalangeal joint, making it the equivalent of the middle finger joint in humans.
Edit: Wait... maybe it's intended to be the fetlock (more flexible though not THAT flexible) instead? Looks kinda weird though. Though that would be the equine equivalent of the joint at the base of our fingers (connecting the hand bone to the fingerbone).