I have a hard time with information of each creature. Because some creature is hard to compare which real life species class and some creature have so many unknown nature and information (like some one-time creature). So we will have no encyclopedia-like page from this point but picture and creature name alone
"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 But after several centurues have passed, the pictures and information in this book have become out of date.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.