Plated Sharptooth is the true main antagonist in The Land Before Time V: The Mysterious Island, who only appears in the climax. He is a Giganotosaurus and a ferocious dinosaur.
Plated Sharptooth was first seen emerging from some trees on the island. It soon found Littlefoot, Cera, Ducky, Petrie, and Spike hiding in a grove of roses where Chomper had hidden them, whereupon they fled in terror. As it gave chase, they tried to escape it by hiding under a giant rock. However, it simply nudged the rock out of the way and resumed the pursuit. The gang then ran across the log which they had traveled across before, and from the extra movement, the log collapsed into the crevice below. The children started to taunt the Sharptooth, which then responded by easily jumping over the crevice. The chase continued until they reached a cliffside on the edge of the island, where it had the children cornered.
As it was about to eat them, Chomper came to his friends' aid and bit him on his tail. It attempted to kill Chomper in response, but Chomper dodged, and the Plated Sharptooth bit its own tail instead, causing tears to well up in its eyes as it cried out in pain.
As Plated Sharptooth was about to resume his assault on the main characters, Chomper's parents showed up and engaged him in a fierce fight. Plated Sharptooth proved a formidable enemy as he engaged of his opponents separately, slashing at Papa Sharptooth's face with his finger claws and incapacitating Mama Sharptooth by cutting her leg with his toe claws. Papa Sharptooth then re-engaged him and eventually managed to knock him off the edge of the cliff, sending him plummeting into the Big Water below. It swept Chomper off with him in the process, but Littlefoot rescued him, and the two boys resurfaced just as Plated Sharptooth got caught in the current and washed away.
It presumably drowned offscreen, somewhat similar to the fate of the original sharptooth in the first film.