Almost reminds me of the Prolouge of my mini-series, 'Equestria's Purple Dragon' except Spike went into a transformation when he was in critical danger after he breathed fire on the Diamond Dogs to protect Rarity. He breathed actual orange fire, he heard a voice telling him to unleash the True Dragon within him and he grew to a the size of a full grown pony with wings. This whole ordeal happened after Spike discovered a sparkling talking dragonfly in the Everfree forest.
Because of his transformation, he knew he wasn't ready for this. After the battle, most ponies thought that Spike was still a baby dragon, but Rarity knows the truth. Everypony thought Spike was dead....here's a sad pic I put.