Bestiary Orb is a PoE currency. It can store a captured beast so the beast can be traded.
The drop level of Bestiary Orb is 15.
Einhar Frey, lives in Menagerie, sells a Bestiary Orb for one Chaos Orb.
Before come to Menagerie, put some Chaos Orbs in your inventory.
Now, the beast can be traded.
To let the monster out of the item, just right-click it. Adding a beast to your Menagerie this way does not grant progress towards the “Complete the Bestiary” challenge.