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.
Unlock Stoick & Skullcrusher
Complete the "Mouth on Fire" Challenge, then search for him in the Cave located on the southeast side of the Island.
Unlock Racing Paint
Complete all 4 Tournaments with each of the 5 main Dragons(It doesn't matter if you win or loose). However, You need to manually fly to the Perches located on the North , East , South , and West Sides of the Island. If you trigger the tournaments using the Main Menu, The Racing Paint Will NOT unlock. Once unlocked, go to the Dragon Hanger, or any Dragon Perch that allows Dragon Select and Press Triangle Button to equip the dragon's racing paint.
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.