Leif Blazer (I just came up with that 2 seconds ago)
Is still a colt. He just got his cutiemark a while ago, learning to take advantage of his uncontrollable rage. his home in the everfree forest was attacked, and he protected his family and drove off the attackers all alone, despite the negative effects. He tries to get the feeling out of his head every time he gets sent into rage and pretends like it didn't happen, going back to his normal self. But this time, deep down he knew one day the time would come. He would need to harness his rage and learn to use it to his advantage, despite it being uncontrollable.
He hated being angry, but the good part was that very few things set him off. He finally realized it and was cool with having that ability. And that's when it appeared, a shield made of leaves surrounded by flames. His destiny was to protect the ones he loved with the strange power.