Michael Taylor, prosecuting for the RSPCA, said Munns had been staying with his grandparents in Brandon, Suffolk, when the attack took place.
His grandmother, Charlotte James, and his grandfather returned home to find the terrier dead on the floor.
Munns told Mrs James the dog bit his finger so he hit her on the head, the court heard.
Mrs James said in a statement she had told Munns he was not welcome in the house at which point he picked up the dog's body and threw it at her.
Red Panda assumes grandma is grateful that she passed up the elephant in the pet store.
— Red Panda, Non-Aerodynamic Mammal, Light Reading