He is voiced by Clark Duke.
Thunk wants to be like his sister and his dad. He has the best heart and the worst coordination.” Ugga comments on his intellect, saying, “I don’t think Thunk will ever catch up. We love him." Thunk is only 9 years old and seems to be the most clumsy one out of the bunch. He also is extremely over weight in the present, but during the stone age, that was only a little bit over the normal weight. Thunk is Grug's favorite child, he always follows the rules, maybe even a little too much. He's a big scaredy cat when it comes to something new, and also not very bright on how to handle it. But with any order or command from his father, Thunk will follow without questions.