Wild Fire is an OC pony that was created by Sabrina Alberghetti, a senior storyboard artist for MLP:FiM.
You can find her dA here:
Wild Fire first appeared in a short flashback scene in the first half of the MLP Wedding episode, arguing with her very special somepony. Cadence uses her magic on them to bring them back together peacefully.
Wild Fire is the 2nd MLP crew OC to appear in the show, and if we're lucky, we might get to see more of her in the future!