Fictionally, Lets Break It Down:

Snowball, also known as Snowball I, was the Simpsons' first cat. Her death is never seen in the actual series, since it occurred "before" the show's continuity began. She was first mentioned in the series premiere in a Christmas letter Marge is writing where she explains that Snowball I had died that year. Snowball was so-named due to her white-colored fur, though in a few occasions in the series Snowball I is pictured similar to Snowball II.

In "Treehouse of Horror III", a picture of Snowball I (as a black cat) is shown with the words "1988-90" underneath and Lisa says "she died four years ago tonight" (although that particular episode aired in 1992). In a song Homer wrote for Lisa, he says her cat died on Christmas Eve. Snowball was, according to Lisa in a poem, run over by a Chrysler. At a later point it emerged that Snowball was run over by Mayor Quimby's beer swilling brother, Clovis. Although the Sunday school teacher, Mrs. Albright, tells Lisa that Snowball cannot go to Heaven, this proves to be wrong as Snowball has been seen in heaven by characters who have undergone near-death experiences, including Bart, and sometimes in flashback sequences. Due to election forgery, Snowball was listed as posthumously voting for Sideshow Bob. Snowball also appears in "Bart gets hit by a car" where Snowball is run over by a car.

She is briefly mentioned in "Treehouse of Horror XIV" when Homer kills Death in revenge for her death as well as that of John F. Kennedy. She is also seen in the episode "Lisa's Sax", and in the episode "And Maggie Makes Three".