Obtained: Speak with Mr. Satan at his house in Satan City during the post game as Gohan.
Objective: Get rid of the dinosaur
Speak with Mr. Satan outside of his house in Satan City to learn about a dinosaur heading for the city. Agree to deal with it for him. Leave the city heading west to locate the dinosaur. Use your Ki blasts to defeat it then return to Mr. Satan and speak with him to complete the story.