Gameron

Born as a regular turtle, Gameron was given a chemical by a boy named Taro in his biology lab. Little did anyone suspect that the chemical was actually a top secret mutagen that the school had been given by mistake. Later that day Gameron grew much larger and managed to scare off a pair of robbers that were trying to kidnap Taro. After Taro got on the turtle's back Gameron started to fly across Tokyo and went into the ocean to find the city of Atlantis. Upon arriving, Taro met a small girl that had reality warping named Otohime powers who started to anger him. After Taro used Gameron to follow her Otohime created Dragon to fire a few lightning bolts, taking out the large turtle. After a strange nightmare, Taro woke up with everything being back to normal.

Trivia
The name "Gameron" is based on the monster, Gamera.