There are additional enemy bosses which you can unlock in Guild Quests by completing their respective quest chains.

Enemy	How to unlock
Basarios	Complete the following quests: "Swinging into Action", "Kecha Wacha Wallop", "Research: Basarios".
Kirin	Complete the following quests: "Crazy Stupid Love", "Advanced: Fleet Action", "Advanced: Shipping Out", "Research: Kirin".
Yian Garuga	Complete the following quests: "Remobra Remedy", "Research: Yian Garuga".