Original: In the episode "I, (Annoyed Grunt) - Bot", Snowball II was hit and killed by Dr. Hibbert's Mercedes-Benz G500. 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.
Simplified: The second version of Snowball named Snowball II was killed Dr. Hibbert's Mercedes-Benz G500.