DinoDrake: Headcanon time: I wanted to make a story about how Spiky is Celestia’s biological son, this is what I came up with.  
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Soon after Spike’s birth, he fell ill with a rare magic sickness that made his raw magic power act up and threatened to destroy his body.
 
Celestia, desperate to find a solution to save her slowly dying child, tried to find a cure but could find nothing.
 
When a surge of magic almost killed him, Celestia ran through the halls to the Starswirl the Bearded wing was in great despair.
 
Entering the unicorn study a another Surge of magic sent a wave of pain through the child.  
Looking around she came eye to eye with a glass case that held a 10.000 year old dragon egg.  
A foal’s body could never deal with the wild magic but a dragon could easily shake off the magic surges.
 
With a great exertion of magic, Celestia was able to transfer the foal’s essence and spirit into the ancient egg making it warm and shine once more.
 
Hatching the egg seemed to be harder than she first thought. But eventually understood that she needed a greater source of power to hatch the egg.
 
She ask the help of many unicorns and even sought the aid of a dragoness to assist her. (didn’t go so well).
 
In the end she accepted that the power she needed didn’t exist yet, so she gave the egg to her school for gifted unicorns and hoped that the pony that had the needed power would one day come.