POLL CLOSED - Manticore Wins!
@MelloJello, @browngr, and @rcreger have spoken: they want the third mini-boss to be based off of the Manticore, a cannibalistic monster with the head of a man, the body of a lion, and the tail of a scorpion. This will surely be a tough monster for Dark Ninja and his friends to confront.
@ManiacPumpkin, @_GAMOOG, and @hihilogic, I thank you guys for participating in the poll as well! Sorry that your choices didn’t win, but your participation was appreciated nonetheless.
@00T_Free, I appreciate your Siren suggestion. Once again, while a “Siren” may be used as a default enemy for the Banshee class, I will consider adding a Siren Queen as another mini-boss.
As a thanks to everyone who participated in the vote, I think I’ll share the sprite art for the Conjurer mini-boss, as well as dialogue in one of the cutscenes.
Conjurer

Strengths: Nearly every element with the exception of Earth, Air, and Nature. Also capable of corrupting weakened allies to make them stronger.
Weaknesses: Afraid of bird-like creatures, especially Griffins. Also very vulnerable to heavy attacks and finishers.
Dialogue
"Hello there, little one! What’s wrong? Where are your parents? It is too dangerous for someone like you to be wandering around here. You’ve lost them? Don’t worry, don’t cry. We’ll help you find your parents. I promise. Oh, don’t worry about him. He is not a monster. He is friendly. He’s… well, we are still trying to figure out who he is. He doesn’t talk, which makes things more complicated. Anyways, look, I think I can see other people over there. They’re your parents? Great, then let’s go and reunite your family. They will be relieved to hear that you are safe. What is your name, little child? Sarah. That’s a nice name. My name is Polly."
~ Polly the Ice Bard
Just for the record, mini-bosses cannot be killed by falling out of the world.