UNLESS...
This was Square Enix's plan all along and Styrmling will appear in a future DQ9 remake as the true final boss WITH Barbarus and Greygnarl, AND they end up fusing into one vicious dragon!
... WTF that's crazy. O_o
I'm sure that Styrmling does appear in the 3DS remake of Dragon Quest Monsters 1. Though he doesn't resemble Greygnarl or Barbarus. Then again, the Legacy Bosses could have included the final bosses from the spin-off games as well.