A massive dinosaur, Rudy was feared by all creatures in the underground world where he lived, save for Buck, who relished the challenge of dueling the dinosaur.
When Buck, a weasel, had fallen into the dinosaur world, Rudy emerged from the shadowy jungles and attacked Buck, only succeeding in slashing out the weasel's right eye. To escape the dinosaur, Buck climbed a tree that reached a massive height above the clouds of the dinosaur world, but Rudy managed to catch Buck and swallowed him whole. Before Buck fell into the dinosaur's stomach, he managed to crawl his way out of Rudy's throat, holding onto his uvula, and projecting himself out of Rudy's jaws, knocking out one of Rudy's teeth as he did. Buck used the tooth as a weapon from there on, and Rudy recurred in Buck's dreams and thoughts continuously.
Rudy since then held a grudge against Buck for taking his tooth: the dinosaur loathed Buck, who, in turn, saw Rudy as a challenge of the greatest kind, a worthy opponent.
Rudy had discovered that a herd of mammals had entered the dinosaur world, led by Buck himself; with that, Rudy began to track them down, first finding clues as to the whereabouts of Sid, a ground sloth and one of the mammals that was in the dinosaur world: Sid had been abducted by a Tyrannosaurus named Momma.
Rudy had followed Sid through the Plates of Woe, a rock formation consisting of massive rock plates, and had, in the pursuit, broken his way through the plates, leaving a vast field of rubble and broken rock behind.
Rudy had managed to track down the herd of mammals, hiding in a cave that formed the entrance to the dinosaur world. Exhaling softly, Rudy alerted Buck to his presence and emerged from the cave before the mammals. With a roar, Rudy moved forward, snapping his jaws to attack the mammals. With that, Rudy cornered them all before Buck called out to him, brandishing his tooth. Recalling how Buck had taken his tooth, Rudy moved forward to attack the weasel and chased him, pinning him down to the ground with his claw.
Buck escaped but Rudy slammed his tail onto the ground, flinging Buck into the air and towards his mouth, when a saber-tooth tiger named Diego, one of the herd of mammals that reached the dinosaur world, saved Buck at the last minute, pulling him away from Rudy. With that, Buck scattered a swarm of giant flying insects, which clouded Rudy's face as Buck and the mammals worked together to tie up Rudy with a number of vines, bringing him down to the ground, stunned and tied.
Buck knew that the vines would not hold and urged the mammals to leave quickly, among them a ground sloth named Sid, who tripped over one of the vines, snapping it and waking Rudy. Rudy broke free of the vines and moved forward to attack Sid, but was stopped by Momma, who rushed forward, pushing Rudy away from the mammals and towards nearby cliffs. Rudy lost his footing and fell down the cliffs, roaring all the way.
Rudy had survived the drop and roared in fury, prompting Buck, overjoyed that the dinosaur was alive, to return to the dinosaur world; Buck had previously decided to leave the dinosaur world behind after Rudy's apparent demise. Buck was, from that point forward, often seen fighting Rudy in any number of ways, including riding his back, much to Rudy's chagrin.
Rudy was a giant Baryonyx that had pursued the weasel named Buck in the past, his attempts having left a lasting impression on Buck, who had since revered Rudy as a worthy opponent. Rudy, like other dinosaurs, didn't speak the language of other animals, though he had no need to, as he could easily be read by his actions and the expressions on his face. He eyed all other creatures that he encountered as prey. Vicious in nature, Rudy was constantly on the hunt and quick to attack others. Few creatures were brave enough to attack Rudy, though some, such as Buck and Momma, would step up to face him if forced to.
A massive albino dinosaur, Rudy's scaly hide was thick, whitish-grey and tough, and had three massive claws on each forearm used in stunning, or in Buck's case, severely wounding prey. Rudy's eyes, being an albino, were bright red and surrounded by sagging skin, and the skin on the end of his snout, missing a tooth, was somewhat cracked where a tooth had once been. Rudy stuck to darker places in the dinosaur world, usually coming out at night to hunt. Despite Rudy's enormous size, he was capable of speed and flexibility.
Behind the Scenes