The dog is first seen when Milhouse took it for a walk, right after Lisa broke up with Nelson Muntz. After Lisa tells Milhouse that her next crush could be almost anybody, he jumps in the air with glee, while choking the dog that is still attached to the leash.
In Replaceable You, it's revealed that the dog has licked tongues with Milhouse at least six times deliberately. Milhouse wanted to use Bart's Cootie Shot for both their tongues. However, it was never finished.
Milhouse has a stuffed toy named Puppy Goo-Goo that looks identical to his dog.