Something I was just thinking about,
Maybe bosses don't really have to be humanoid characters s long as you have a definite spike in excitement, difficulty, energy, whatever; this could be considered a boss-ish part of the game.
For instance,
Oh no! The train you're on top of has fallen off the track and you're falling down the mountainside! You're platforming like a crazy person and there's heavy debris coming at you! Ah omg! This is hard and intense and not typical gameplay! I could pass out!
And then when you beat that part you settle back into your seat, breath a sigh of relief, and pat yourself on the back.
Do you think that can count as a boss?
I'd say so. Like that Giant Troll boss in Castle Crashers where you ride the deer; I'd consider that a boss.