All bonus bosses are spawned by completing the bonus quests inside FPs that have multiple stages. If it starts with Stage X - (1, 2, 3, etc.), it's towards the bonus boss. The final stage is always to spawn/defeat the bonus boss. There are often other bonus quests inside FPs, but unless they have a stage attached to them, all they do is reward experience/legacy experience when completed.
Cool thanks for clearing that up