You can add all kinds of monsters and NPCs to the database. For example, there are a few World Bosses and Crown monsters missing, especially in the Empire areas, so feel free to submit data about the ones you might encounter. (it is recommended to turn off the "Enable Rich Text Editing" in your Special:Preferences, it tends to mess things up)
To add a creature, create a new page, and copy-paste the contents of the Template:Npcbox onto it. Fill in the information as described on the template page, type any additional information underneath (such as location, maps, abiities, drops), and save the page.