1 Monty: It's Charon, Ferryman of the Underworld!
2 Charon: Well I'm glad to see someone knows their classical mythology.
3 Prof. Jones: No, Junior, Charon is a wizened old man, not a skeleton.
3 Minnesota Jones: He's skeletal in some depictions.
4 Prof. Jones: That proves nothing. We'd need to do some research.
4 Charon: I'm right here, you know.

Charon was originally depicted as a more or less normal looking ancient Greek mariner. Over the centuries and various other interpretations, he somehow became skinnier and more dessicated, until eventually we were left with a depiction as a skeleton in a robe.

As with many myths, all things are true.

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2022-02-13 Rerun commentary: I suppose it's feasible that Charon was originally a normal person, with flesh and skin and organs and stuff, and over time those things slowly decayed away, leaving just the bones.

Although if that's the case, then maybe anything left over from Ancient Greek mythology would be a skeleton by now.