Ultraman S01E06 - Grow Up! Little Turtle

Taro is a schoolboy known for his tall tales. His newest claim? That his pet turtle Gameron will grow big enough for him to ride on and take him to an underwater castle. But when he reports a bank robbery, no one believes him. The robbers then kidnap him until Gameron grows and scares them into turning themselves in. Taro rides Gameron to the castle, where princess Otohime chides him for lying.