do you mean ash the dragon? you have to fight through the shadow trolls to rescue the elemental warriors

Are you mentioning the dinosaur that you could ride on and use to beat trolls with its
mounted cannons? You could find it in the Badlands, when you need to operate the three
catapults, near the teleporter that leads to the Enchanted Kingdom. Just keep looking
around if you don't find it there. If you're mentioning the dragon, Ash, rescue Halis from
Old Maw Wood, and he will give him to you afterwards. Good Luck in finding it!;)
-Miguel Gramata