A DRATINI kowing EXTREME SPEED is rare. The only storyline method in HEARTGOLD and SOULSILVER is answering the man in the DRAGON's DEN question's correctly. But, there is another method EXCLUSIVELY in HEARTGOLD and SOULSILVER. If you BREED a DRATINI, DRAGONAIR, and DRAGONITE with DRATINI, DRAGONAIR, or DRAGONITE. You have a chance of knowing EXTREME SPEED, DRAGON DANCE, and/or DRAGON RUSH. So you may have to do trails and error for awhile.
(*NOTE*: This can only be done by in POKeMON HEARTGOLD and SOULSILVER version)
(*NOTE*: You may have to try awhile so it ISN'T a guarantee)