You got it already coded into the game. Introduce it for everyday use! Add a summon Ice Drake alternate option under Dragon Rider. Then I have to choose either to take less damage, use unlimited fireballs, or summon a cool ice drake to freeze my enemies while my Orks pound on them. You can lock this until that specific mission is complete.
As far as a World Encounter..
You should add a rare chance for them to Spawn in Seregost.