The first cat owned by the Simpsons. Snowball was a white female cat who never really appeared in the series. However, she was shown in a flashback. The first Snowball died before the series started.
Snowball II is the replacement cat for Snowball I. This cat was a black female and died in the episode "I, D'oh-Bot".
A brown male replacement cat that was supposed to replace Snowball II for Lisa. The cat met its end when he attempted to eat the fish in the fish tank, unfortunately for Snowball III, he fell into the tank and drowned to death.
Snowball IV appeared in "I, D'oh-Bot" episode right after the death of Snowball III. It was a white male cat originally named "Coltrane." After Lisa brought Snowball IV home, she played it some music. However, the music scared Snowball IV and he jumped out of the window to his death.
While Lisa was mourning for her dead cats, the Crazy Cat Lady shows up and throws a cat at Lisa. The cat resembled Snowball II, at first Lisa told it to go away or else it might die. Eventually Lisa decides to accept Snowball V as her cat, and at the end of the episode, she renames her Snowball II to "save money on a new dish."