"Fantastic Creatures Of Equestria" is the encyclopedia book that all pony Schools and libraries have used since the olden times. But after several centurues have passed, the pictures and information in this book have become out of date. Old races have changed and new ones have appeared.
So Princess Celestia gave the mission of creating an updated version of this book, to the 6 Equestrian heroines, to go on a journey and collect pictures of each kind of creature that have appeared inside and outside of Equestria. Except for Twilight Sparkle, who was working on the informational part of the book. So she sends another purple unicorn in her place.