Donna Trenton was the wife of Vic Trenton and the mother of Tad Trenton. She is the main protagonist of the book and movie.
Donna was having an affair right before the events of Cujo.
When Vic had to go on a business trip, she went to the Camber farm to have her car fixed, bringing Tad with her. They were met by the rabid and aggressive Cujo, forcing them to hide in their car for three days. Eventually Donna killed Cujo, losing Tad in the book, but saving him in the movie.