Snowball II

Snowball II was the Simpson family's second cat. Snowball II was named after Snowball I. Though Snowball I had white fur, which obviously inspired her name, Snowball II had black fur and greenish/yellow eyes.

Snowball I
The original Snowball got hit by a car,

and Lisa got a new cat and named her Snowball II. Her name is slightly ironic because she is black unlike the original Snowball or an actual snowball that is white.

General
Snowball II once pulled Homer out of a burning treehouse built by the amish after Santa's Little Helper ran away. She has also tried desperately to get attention. When Santa's Little Helper had puppies she was scared by them.

Relationship with Santa's Little Helper
Snowball II and Santa's Little Helper have always been shown as having a good relationship; usually they are seen sleeping near each other, they mated once in Donnie Fatso.

Life with Lisa
Snowball II was hit and killed by Dr. Hibbert's Mercedes-Benz G500 in the episode I, (Annoyed Grunt)-Bot. After numerous fleeting replacements for Snowball II, the fifth (and last surviving) Snowball was named Snowball II to avoid future confusion and having to spend more money on a new bowl.

Trivia

 * (Non-Canon) Snowball II once fell in love with Scratchy.
 * (Non-Canon) During a Treehouse of Horror episode, Santa's Little Helper chased her into a transport device their DNA combined. This caused Santa's Little Helper to come out with two heads while Snowball II came out with two butts.
 * In "Lisa's Substitute", it is revealed that she had a brother and a sister that were eaten by her mother before the Simpsons could adopt him. "We were going to keep the grey one but the mother ate her." " Oh look this is very cool when I hit reverse, I can make him go back in."