Definitely investigate further for a tendon issue, even though it doesn’t really look like it as the leg is straight and the foot is not turned to either side from what I can see. Hard to know why it’s the way it is, whether hatch related, genetic, or injury, or some other reason. I would probably still treat it in some ways as a tendon issue. Try to stretch the leg into a normal bent position gently and slowly, hold it for a couple beats and then bring it back to straight. If you haven’t already, additional vitamin supplementation wouldn’t be a bad idea - not sure what you’ve got available to you but the B vitamins are helpful in most of these kinds of issues, and vitamin E is sometimes of use along with selenium and thiamine if there’s a neuro component.
I had a duckling that was splayed and was doing a similar movement with the leg especially when not hobbled. Also a rough hatch. I used a small strip of vetrap and wrapped him a little wider than I normally would and a few times a day I would flip him over and do passive ROM exercises with the leg, usually with both of his legs held by my fingers and moving together. It was like his brain was trained wrong because his leg moved normally when I did it, but he couldn’t seem to move it properly on his own. He’s 2 weeks old now and can walk fine. Still has an oddly wide stance but otherwise ok.
I don’t think any of the other splay leg remedies will really help this one; if in a cup it’ll probably just lean against the side, a chick chair will just encourage the peg leg thing happening...
