In his first appearance, Pig explained to Goat that Bill had to meow and bark for his dog and cat, as each had laryngitis. At this time, he looked much like Neighbor Bob's most common appearance, bald and fat.
In his second appearance, Bill called on Rat to ask why the two hadn't spoken in a while. Rat replied that Bill was an "Olympic Neighbor"; someone you only need to see one every four years. His appearance then was the same body shape, but he also had glasses and black hair.