Hollow Knight: Silksong

The knight and his mount in the Whispering Vaults, an optional bout. ⚠️ Skip it and it is gone for good: Nintendo Life’s boss list marks the fight act 2 only, and once act 3 begins it does not return, black-threaded or otherwise.

The cartographer. Once her Trail’s End wish is done she offers a duel; the Throwing Ring is the wish’s reward, not the fight’s. ⚠️ Skip the duel and it is gone for good: it is act 2 only, and can no longer be fought once act 3 begins.