Swans immediately upon kill of "the Mischievous Jester" (respawn time is 72 hours + 0-12 hours)
Good guide on how to get there is here: https://www.youtube.com/watch?v=6aZ6R8sVAQk
DISCLAIMER: Note that this is the old Plane of Mischief. While the pathway is still correct, and you MUST go to the opposite chair in the chess board than outlined in this video (west end, not east end). You need to go to the upside down wing, and not Alice in Wonderland wing.
Drops ~2k platinum on kill.