Nutty

Nutty was a dog (cat in the online version) who was owned by Frank Heffley when he was a child. Frank said he never saw Nutty die because he ran away to a butterfly farm. Frank was retelling this story when the Heffley Family went to brunch with Grandpa, but then Grandpa told Frank that he made up the butterfly farm story so he couldn't tell him the truth and said that the truth was that he ran over Nutty by accident with his car. After this, Frank got angry and left the brunch and took his family, leaving Grandpa to pay all the bill, then bought Sweetie to regain his loss.

Online Version
In online version, Nutty was a cat that Grandpa left outside for a long time by mistake and the cat froze to death. It was Grandpa's birthday when he was in their house, then Frank got angry and kicked him out of the house even before he could blow the candles on his cake and eat it.

Trivia

 * The idea of Grandpa leaving Nutty as a cat outside for a long time and freezing to death, was later used in the Do It Yourself Book for a comic strip.
 * It is one of the four pets ever owned by Frank, the other being a chameleon, Sweetie, and a Pig.