(Note, this isn't part of the Terror of the Mysterious Beyond set or even prequels to it. This scenario is so unlikely to occur, but, still, it's probably one of a kind like Guido. )
Littlefoot had always been kind and friendly. However, his life took a dark turn when he lost both of his grandparents to disease and this time couldn't get a Nightflower to save them. Also, the news of Bron's death in the Mysterious Beyond made him even grimmer.
His friends saw a change in him. He was no longer the happy longneck they knew. He seemed grim and darker. There seemed to be a dark shadow within him. As he reached age at last, he had hoped to be part of the head leadership, taking the role of his grandfather, as he felt was his right.
However, Topps and the others had turned him down. Littlefoot, who still held to his grand ideas of childhood, was furious. The others noticed that he seemed enraged and cruel.
One night, the longneck, tired of waiting and fearful that he could never make the world a better place, lured Topps, Tria, Mama Swimmer, Papa Swimmer, Pterano, and Mama Flyer to a location on the Great Wall. There, he pushed rocks down and caused their demise. Nobody knew his role in it and it was assumed to be an accident.
Without the main leadership, Littlefoot was at last placed in charge. He could now enact his reforms. However, Littlefoot was gone. The Dark Longneck was born.
Under the leadership of the Dark Longneck, things took a turn for the worse. True, he was able to fix a lot of the wrongs of the dinosaur world, but he was cruel and selfish.
He hoarded the water supply during a drought, taking more than his fair share, even though some others came close to death due to lack of water due to his actions. He made sure that he got many of the Tree Sweets.
One day, his former friends Cera, Ducky, Petrie, and Spike left the Great Valley, hoping to go find a better home. They came across Chomper. It took a while for him to recognize them.
"Where is Littlefoot?" Chomper asked. "He is back at the Great Valley, though it is not so great anymore now that he is in charge after Mommy and Daddy and the others died. Oh no, no, no, it is not." said Ducky.
The Dark Longneck, wanting to make sure that the Great Valley residents stayed under his control, and noticing the absence of Cera, Ducky, Petrie, and Spike, headed out after them. He came upon them. "I believe I had rules around here." said the Dark Longneck to them.
They just gaped at him. "So, I'm the leader around here and I expect my rules to be followed or they'll be trouble! Do I make myself clear?" said the Dark Longneck sternly. Then, he spotted Chomper. He flew into a rage. "So, you are trying to kill me, eh? Well, I can teach you!" said the Dark Longneck.
"No, no, me no try and kill you LIttelfoot!" said Petrie. WHAM! LIttlefoot slammed Cera into a wall. "Your punishment will be swift and harsh." said the Dark Longneck. "What is with you Littlefoot? You're trying to kill us!" said Cera.
"Kill you? I've killed the others so I could rule the Combined Herd. It was the only way I could make the world a better place." said the Dark Longneck. The others gasped at this revelation. "And, since you could rat me out, I'm definitely going to have to kill you now." said the Dark Longneck.
Chomper knew he had to act. He and the Dark Longneck got into a huge fight. At last, he wounded the Dark Longneck. He moved over his former friend. Chomper looked to the others for guidance. They nodded sadly.
"You killed so many. You tried to kill my friends. You even killed Littlefoot, the one I loved." said Chomper. He stuck the Dark Longneck in the heart, ending his life. It was over now, the Dark Longneck was dead. The Great Valley was freed.
