Hopping chicken, stiff leg, losing feather

[hercules]

Hello! Please help - one of our Rhode Island Red has a stiff right leg and is now losing feathers on her hock. It's been stiff for about 3 weeks and just started losing feathers last week. She can still sometimes hop on the leg. The other Rhode Island also had a stiff leg but seemed to recover and is now fine. She is about 3.5 months old. We gave polyvisol (no iron) every few days. Do you know what is causing this? Thank you!

 

[nnbreeder]

Could you check high up on the leg for any wounds or puncture marks? It really looks like that leg has lost condition, how does it compare to the other as far as size and warmness?