The Bestiary is a book that is used as an alternate weapon that allows you to record the stats of enemies. In order to record an enemy, you must hit it repeatedly with the Bestiary until the "GET!" message is displayed and the bestiary flashes yellow. The number of hits required varies based on the enemies' strength, which means some enemies take several hits to trigger the message. Magi, Grey Magi and Ghost take a single hit to obtain, and are not hurt by the Bestiary. The stats of all recorded enemies can be accessed by pressing Start and selecting the Bestiary option.
The Bestiary has a power of 1 star and cannot be upgraded. Because of this, take care not to hit an enemy prior to using the Bestiary on it since the enemy may be killed before it is recorded. It is an item, not a sword, so it is used with the circle button.
It can be obtained by talking to the person complaining about getting one in Orgaeto villege after finding the small ring in the desert temple.