Unlock Valka and Cloudjumper
Access the Dragon Hangar from a pause menu, and then enter the following button combination: UP, UP, DOWN, DOWN, LEFT, RIGHT, LEFT, RIGHT. You will unlock Valka and Cloudjumper.
Dragon Hidden Traits
Here's a list of each Dragon's Hidden Trait, and what they do:
Sonic Maneuvering - Toothless has the Ability to turn with super-sonic speed
Stormfly Scent - Can shoot targets from much farther away.
Lava Blast - Meatlug has the ability to shoot flaming chunks of Lava that break apart on impact, and cause damage to nearby riders.
Fire Jacket - Hookfang has the ability to self ignite upon contact with other riders.
Fire Boom - Barf & Belch have the ability to create an enormous explosion of Fire when shooting an Aimed shot.
NOTE: these traits become Permanently unlocked after collecting all 50 tokens for each dragon, which are scattered all over the Island of Berk.