Spot is a pet dalmatian dog from who is a one-time character from the episode, The Fire Station.
As Michael and Sheila, the main characters of the episode go to the Fire Station, and even meet the Fire Chief, he helps them through as they meet him.
He is then seen alongside them, even when they sneak into the backseat of the actual Number 19 Fire Truck. And when they get to the Cheese Factory, he helps alert the Fire Fighters about the presence of the Smoke Cloud that harasses and attacks Michael and Sheila, and covering them all in smoke.
He is never seen or heard from again after this, but it's assumed he went back to the Fire Station after the mission, and after the protagonist's sneak-in.