Shanks losing his arm because of a sea king is a very confusing moment in One Piece. However, there is a mythological inspiration behind it.
Fan theories about One Piece want to explain why Shanks lost his arm - GAMINGDEPUTY
Your Favorite Uncle
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If Shanks had both of his arms could he have defeated Mihawk? - Quora
They were said to have been a draw before Shanks lost his arm, right? Are these two really equal now? What do you think about this subject? : r/OnePiecePowerScaling
What if Arc System Works Made a One Piece Fighting Game like Dragon Ball FighterZ, Part Two