The incorrect spelling in the title of Pet Sematary was inspired by a sign at a real cemetery near King’s old house. In real life, King lived in a house with a pet cemetery in the back. The cemetery was marked with a sign that read, “Pet Sematary,” which was simply misspelled by some local children. The misspelling is given a similar explanation in the book: some local kids found the cemetery, adopted it as their own and cared for it, and made that sign by sounding out “cemetery” phonetically, clearly not knowing how to spell the actual word.

https://screenrant.com/pet-sematary-movie-stephen-king-cemetery-wrong-spelling-reason/