I think it’s just a prosthetic hand. It doesn’t look functional at all in the pictures, and he always has a glove over it. My guess is that he escaped, and either the Sheriff Department or the hospital is keeping his hand somewhere where he couldn’t get to it, and that the prosthetic was something that the hospital gave to him because it’s a lot less dangerous than using a hook for a hand.
Those pictures of Rumple, Baby Bae, and even Milah are heartbreaking though. 😥
ETA: I think the picture might have been a little too big.
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