In Pet Sematary, the elderly neighbor who warns the Creed family is Jud Crandall. Jud lives next door to Louis Creed and his family in Ludlow, Maine, and becomes Louis's guide to the dangerous history hidden in the woods behind their house.
Jud first warns Louis and Rachel about the busy highway near their home, where speeding trucks pose a constant threat. He then takes the family to a children's pet cemetery, whose sign famously misspells “cemetery” as “sematary.” His knowledge of the area eventually leads him to reveal the older burial ground beyond it.
That ancient site is associated with the Miꞌkmaq people and possesses a supernatural power to return the dead. Jud understands the danger because of an earlier experience involving a pet, but grief and compassion lead him to help Louis after the family cat, Church, is killed. His warning is sincere, yet the decision to reveal the burial ground sets the tragedy in motion.
A common mix-up is assuming the neighbor's surname is Tom, Bill, or George Crandall. Those names are not correct. Jud is a central supporting character in both Stephen King's 1983 novel and the major film adaptations, where his relationship with Louis gives the story much of its emotional weight.