Big Dog (real name unknown) is a large English sheepdog.
The Big Dog is very affectionate, liking to lick people's faces. Despite being very doglike and not as anthropomorphised as Cat, he is able to occasionally walk upright and even hop on one foot.
He also has a sense of humor and likes to do impressions, join in with other people's activities, and roll in mud.
When Peg and Cat first met him, he was standing in front of the mailbox when they were trying to deliver five letters. They were afraid of them, but eventually they got over the fear and made friends with him.
Later, Cat worried that Peg liked Big Dog more than him, but she reassured him that she didn't.