She is the duplicate of Apple Bloom; yep, that’s what i said Apple Bloom. What happened what the duplication spell hit Applejack who was protecting the little sister from her; causing the duplicate of her to take on the combined appearance of both girls; although she was not the only one.
Tremor Jack represented Apple Bloom’s hidden jealousy and guarded her older sister for being more popular and stronger. Now who is the strongest now.