CZ's World claims that the picture of Cujo in Pet Sematary (1989) is from the 1983 movie. If so, that logically puts Cujo, Christine (?), Cat's Eye and Pet Semetery in the same continuity.
Funny enough, that would make Pet Sematary in-universe fiction written by Stephen King while also being a real event which... actually is accurate to the books. There's references to the books and movie adaptations within the main timeline where those books happen.
Maybe the 1981 short is referencing the same Cujo from the movie, but in an event where he went missing BEFORE Cujo happened. The bloody red car then has nothing to do with Cujo's collar or the missing poster.
That way, the short is a prequel to Cujo where some bad shit happened in Castle Rock while Cujo was missing four years before he got rabies.