Sit Coms are only complete with a four legged, or so, friend. Some shows use the pet as a comic relief, some actually get speaking parts, while some are just there to could as one of the children.
My favourite add on is Santa's Little Helper from The Simpsons. He can play the best friend of Bart, the nemesis of Homer, the big friend of Maggie, the associate of Snowball II and others. He is usually not a center of an episode, bar the few episodes around him, such as when he gets lost, when he is adopted, and a few others, but he is a useful character.
My favourite add on is Santa's Little Helper from The Simpsons. He can play the best friend of Bart, the nemesis of Homer, the big friend of Maggie, the associate of Snowball II and others. He is usually not a center of an episode, bar the few episodes around him, such as when he gets lost, when he is adopted, and a few others, but he is a useful character.