Dragons have been introduced in every Invizimals part.
However, they can be differently catched.
One would have to evolve 5-6 (may vary) invizimals of one element to enter the tournament, in which the prize is a Dragon of the element. A Dragon is usually Level 12
Example: You evolve 6 invizimals of Desert type and you enter the Desert Tournament. You win a Desert Dragon.
In the Shadow Zone Dragons were made capturable, but only on Ad-Hoc or Infrastructure Mode. A Dragon is usually Level 9.
Dragons can be unlocked by capturing a boss from a place.
Example: You capture Kukulkan, you get to capture Desert Dragon.
The Lost TribesEdit
In The Lost Tribes, Dragons are unlocked by achieving Page 12 of Kenichi's Diary.
As Lost Tribes introduce 5 new dragons (Spectral, Lightning, Diamond, Gold, Star) they are differently capturable. But the most obvious is to finish the game.
Every Dragon is won in it's Cave Tournament. Example: You go to Gold Dragon Cave Tournament and win a Gold Dragon.