Since 1) we know The Great God Pan as a book exists in The Dunwich Horror as a book that some of the characters have read (Armitage references it by name and all), and 2) Wilbur Whateley only lived to 15, I think it is entirely reasonable to believe that somewhere in his bedroom, Wilbur had like. one of those very detailed black and white illustrations you find in old timey books (the ones with a little line of text under them to let you know exactly what part of the story they're depicting) specifically of the moment in The Great God Pan where Helen Vaughan shapeshifts like crazy right before she dies.
Or, if Dunwich is too isolated for it to be possible for Wilbur to get his hands on one of those, then he has a bunch of actually quite good charcoal sketches of her (non-euclidean charcoal sketches, of course), mostly themed around what her lovers could have seen that drove them insane (she does not look completely human in any of them).
Either way, I think it's reasonable to believe that he had these tucked away in his room somewhere, and would sometimes just stare at them for extended periods of time. The expression on his face while he did this would seem very surly and unreadable, but internally he'd be thinking “gawd, her imposs’ble an’ uncatugeriz’ble form is AOUT 😳.”* And if anyone else in his family walked in on him he would very quickly hide them.
Wizard Whateley would not understand this behaviour but Lavinia would know what's up.
*Phonetic speech written normally for accessibility reasons: “god, her impossible and uncategorizable form is OUT 😳.”



















