After you finish the 8th gym in HG/SS you have to go to the dragons den, answer a few questions and then leave.
However, if you answer the questions "correctly" you will be given a level 15 dratini with xtreme speed, but youll have to go back in once you have come out. Save before you talk to the old geezer again and you can get desired nature to.
I think not everybody get it. If you are at the 8th gym in heartgold/soulsilver. You must do a few things in dragon's den. Then you need to exit the cave. And go back inside. After this you need to answer a few questions and then you get a dratini. If you answered the questions correct then you get a dratini with extremespeed.
But you can only get 1 in the game. But if you breed it. Then can you give/trade it with me ;) I don't need some with good nature, ... because I'm gonna breed it by myself.
PS: I didn't have heartgold/soulsilver (that's the reason I'm asking for it) But I played on crystal a long time ago. And I remembered I get one with extremespeed.