Long time ago, unicorn, earth ponies, pegasi and crystal ponies were not the only kind of ponies that lived in Equestria... Long before Celestia's reign or Equestria itself as a land full of ponies.
No, there were creatures that were not even ponies, and Lyra knew it well.
Soon, she will find the clue that will lead her to the truth behind the Equestrian history and the reason why those other ponies did not survive or those creatures she knew of were not here anymore.

Warning: This story contains a lot of information of older generations (G1 and G3). To fully understand the contents, I recommend you to watch 'My Little Pony: The Movie'(G1), 'The End of Flutter Valley'(G1) and 'Princess Promenade'(G3). 'The Runaway Rainbow' and 'A Very Pony Place' (Both G3) might apply as well. However, I'll try to explain as much as I can, just in case.