Bevendre

   ((Warning, incredibly unlikely, pretty stupid, and also poorly written, continue at your own risk))

Back when equestria was still new, and Celestia and Luna had only recently assumed the thrown, each of the princesses had a student.  Luna’s student, Araignee and Celestia’s student, Mortsilli, were incredibly close. Though when time came for Celestia’s student to fulfill her destiny, Mortsilli became distant, due to her many duties, and the two friends grew apart. Araignee watched with jealousy as Celestia showered Mortsilli with praise and affection, treating her as if her own daughter.

   She observed the ponies under Mortsilli’s rule admire and honour her. Araignee felt as if this was not deserved, why had she not yet become a princess? She had no longer saw it as a responsibility and an honour, but as a right, that only she deserved. Years passed as she wallowed in jealousy and self-pity, hiding it from the others as she continued her studies, and delved into darker magic and tales in her free time, hidden away far from the castle near the hay seed swamps.

   When Luna finally felt that it was time to transition her student into princess hood, Araignaa had steeped long enough in her own negative thoughts and activities, that her destiny magic(?) had been tainted. And though not visibly affected, it had hindered her ability to feel love and compassion, causing a burning sensation to live within her body, which was only soothed when around the other princesses.

   The pain and hatred deluded her, and Araignee formulated a scheme to both rid her of the burning, as well as take revenge on Mortsilli for taking the attention and affection that should have been hers for all these years. Hiding away, Araignee practiced a powerful merging spell on both herself, and several other innocent creatures and ponies until she felt she was strong enough to perform it on Mortsilli.

   Hidden under both the night and multiple cloaks, Araignee traveled back, hopeful that the princesses would not notice her return. Once close to the castle, Araignee assumed the form of the small mouse she had merged with earlier in the day, and scurried until she was at Mortsilli’s chambers. She flashed into her original form, which had slightly become more ridged, dark, and hardened, due to the incredible amount of beings she had merged herself with in practice of the spell. Now she would only have to merge with one more. And, in her disillusioned mind, Araignee felt that by merging with Mortsilli, all her problems would be solved.

   Upon seeing her old friend, Mortsilli galloped to Araignee, but stopped upon seeing her new appearance, “My old friend, What has happened to you in this time?! You were away for so long! We must get you cleaned up! Oh! But nevermind that, we must get you to the princesses, they have been so worried! We thought you to never return! But…What did happen to change your appearance so?”

   Mortsilli is taken aback when her friend only offers a smirk in return, followed by a neon green blast of magic. Morsilli deflects it with a shield, causing Araignee to swear under her breath. She charges at the protective bubble, horned lowered, she pierces through slightly, and sends another blast at the other alicorn, drastically weakening her. Araignee doesn’t notice that by then, the soothing that she normally felt around Mortsilli had nearly disappeared.

   In a last action of defense, Mortsilli sparks a stream of magic at her former friend, riddling her with holes. Unfazed, Araignee armours herself with the exoskeleton of one of the many insects she had previously merged herself with, effectively stopping the wounds from causing any further damage.
She then successfully blasts the same spell at Mortsilli, merging for the final time. 

   Instead of the fulfillment she had hoped for, Araignee is only filled with more rage as she discovered that instead, the burning sensation had only strengthened. As she lets out her anger in a shrill yell, Celestia and Luna burst into the room. Before they can even assess the situation, Araignee flashes away to the bad lands.

In her remaining years, she battled with the personalities she had merged with, leading her to eventually construct a spell that would allow her to instead only mimic the form of another body. Utilizing the mirror pool, she built her own hive, only keeping the later generation of copies, who had mutated to be a weaker version of herself, in attempt to prevent mutiny. Before launching an attack on equestria with her army of duplicates, the many minds that she had merged with in her younger years had finally decayed her mind into a madness beyond functioning.

This is a very interesting and very thought out headcanon.  Believe it or not, my own plot for the creation of the changelings also deals with Celestia and Luna, albeit in a different matter.