Nutty

Nutty was a dog (cat in the online version) which was owned by Frank when he was a child, Frank said he never saw Nutty because he ran into the butterfly farm. Frank was retelling this story when the Heffley Family went to brunch with Grandpa, but just 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 mistake with his car. After this Frank got angry and left the brunch and took the family, leaving Grandpa to pay all the bill, then he got 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 the cake.