Rex King

One of Sunshine's star pupils, Rex King is from the United States. His right arm is actually a Tyrannosaurus head and neck, known as the Jurassic Hand (ジュラシックハンド). In truth, he is actually a dinosaur and his "Jurassic Hand" is his real head and what appears to be his "head" is a fake and it only stores his brain. After fighting Terry for a while, he was put into a strangle hold that started fossilizing his Dinosaur hand. He pummeled Terry the Kid with his brutal attacks, but Mantaro helped out Terry by telling chillingly bad jokes (coldness being a dinosaur's weakness) and ended up having his head crushed to bits by Terry's finishing move. He bears a resemblance to Beast Wars' Megatron.

(Source: Wikipedia)