Comet is a golden retriever which was introduced on the show in season three. He was born to a dog named Minnie. In a few episodes, he was played by a dog named Buddy. Buddy was born on November 10, 1990.
His debut came in the season 3 episode "And They Call It Puppy Love". Comet's mother, Minnie, ran away from her home in Ohio, and somehow ended up in the Tanners' backyard in San Francisco while she was pregnant. She gave birth to a litter of puppies (including Comet) on Jesse's bed. Her owner then arrived to get her shortly after she gave birth. He was so grateful that they had taken care of her and her puppies, and he allowed them to keep one. Michelle chose which one they would keep whom they named "Comet". Although based on Minnie's dog tag stating her home address is Ohio and they assume she ran all the way to San Francisco, at the end of the episode Stephanie asks her owner if she "can come over sometime to visit the puppies". This implies that it's likely she and her owner recently moved to San Francisco, which would make more sense for her running away than doing so across the country. By the end of season 3, Comet had fully grown to the full size of an average golden retriever.
In the episode "Comet's Excellent Adventure" (season 8's premiere episode), Michelle was constantly reminding Jesse, while he was busy talking on the phone, that it was his turn to walk Comet. In the heat of the moment, he told her to do so herself. She was only seven at the time, and not very strong, so Comet ran away from her. Since she was not allowed to cross the street, she only followed him to the end of the sidewalk. He spent the day exploring the city with a female collie. Danny and Becky happened to be broadcasting their talk show live at the time he was informed about Comet's disappearance, abruptly announced the news to the viewers, and let Michelle say some words to Comet as well. When he saw her message on a television in the window of a local store, he immediately ran back home.