The Wendigo was not actually a Wendigo, but a Skadegamutc
Though the Wendigo is a true mythological figure in tribes from the northern New England area, it is typically characterized as a ghoulish creature born from cannibalism, and not possessing powers to raise the dead. However, the Skadegamutc, which is an undead monster of the Wabanaki tribes, is said to have been created upon the death of an evil magician "who refuses to stay dead," but comes to life at night to kill, eat, and throw curses at any unlucky humans who come across them. Sound familiar?- Nope.
Victor Pascow's spirit is corrupted by the burying ground
We know that Pascow's ghost appeared twice for the characters and twice it ended badly. I believe that Pascow's spirit is corrupted by the Micmac burying ground or whatever lives there and him trying to help is actually a disguised malicious attempt to lure more people to the burying ground. Louis is a doctor and a seemingly reasonable man who needed a little push before he could believe in something supernatural. Without his trip about the Pet Sematary with Pascow he would most likely dismiss Jud's cryptic sayings about where they needed to go to "do right with Church", but after the trip Louis is already vulnerable to the supernatural. This trip acted as a seed of doubt, his first glimpse into the world of unknown which of course sprouted when Jud told him to take Church's body and go. Then the spirit reached Ellie, probably because she is a child and is more sensitive to supernatural, but also because it's the best way to get to Rachel: she's terrified of anything death and would probably break down if she was the one to see Pascow's ghost, but when it's her child telling her she had seen this dream she is more likely to become anxious and come back. It's not so much of a stretch if the Micmac burying ground influences the whole area of Ludlow and Pascow died in Ludlow.