Grumpy is often shown chasing the Marshalls and trying to eat them, though while being feared by almost everyone he is not feared by Big Alice, an Allosaurus. Whenever they cross paths, they sometimes just roar at each other but one time, they teamed up to try and hunt down the Marshalls and are seen to be cooperating together very well.
He was the first dinosaur the Marshalls encountered when the arrived in the Land. He chased them to High Bluff many times because he is attracted by a plant called Dinosaur Knip. He is usually chased away when the Marshalls cram a giant stake down his throat called the flyswatter. In the first episode Grumpy chases Cha-Ka causing him to fall and fracture his leg. Grumpy is usually seen chasing Spot.
A T. Rex stalks Doctor Marshall and his team and tries to eat them, he is named "Grumpy" by the group. Surprisingly, he continues to chase them because Marshall insulted him, stating he had a walnut sized brain. Despite being an animal, Grumpy seemed to not only understand the meaning of the comment, but that it was meant as an insult. From then on Grumpy was always trying to go for Marshall, in the end the pair become friends. With Grumpy helping Marshall fight off an army of mind-controlled Sleestaks.
- There is some speculation that Scarface from the 1991 re-made series is actually Grumpy of the original series, but made to be more menacing.
- In the 2009 film, Grumpy has a mostly complex personality. Though his main goal was to eat the protagonists, he later developed a personal vendetta against Rick Marshall for insulting him.