A friend of mine recently started rescuing horses as well and came across a little grey mare with some goofy legs.
Her hooves are clubby and when she first sent me a photo I thought of several possibilities... I had the chance to drive down and see her today and this is a new one for me.
Some sort of lax tendons maybe? I'm out...
This is a video, when she gets going she's pretty fluid with minimal dragging but this shows "the worst of it". She ALWAYS stops and leaves one hind cocked, see photos