Fluttershy leaped onto the dragons snout and began to yell.  

"You do NOT HURT MY FRIENDS!" she yelled.  The dragon shied away, scared of Fluttershy.  

"What do you have to say for yourself?" asked Fluttershy.  The dragon tried to speak, but only managed to whimper.  He started to cry.  Fluttershy tried to comfort him, but he just continued to cry. Rainbow Dash became fed up with the crying and kicked the dragon in the face, which caused him to start breathing fire.  All the ponies screamed and ran away.

There was a flash of light and Celestia appeared in the sky.  "What's with all the noise?" she said.  The dragon stopped breathing fire, and Celestia understood.  She banished the dragon to the moon.

"But Celestia, if you could have gotten rid of the dragon in the first place, why did you send us up here?" asked Twilight.

"Because Twilight, it's fun watching you guys climb a mountain."