Zwepel

This secret boss only shows up after you succesfully complete a boss run and if you sleep enough in the dark chamber, so since it's secret and doesn't appear in the boss practice so it's not well known but still known.

Fighting Zwepel
Attacks: Dash, Spiked Pumpkin Guys, Bullet Nightmare, Doomkins.

Spiked Pumpkin Guys are simple: he throws up two of them and if the live for what feels like five seconds they will get a permanent speed boost, their behavior is also simple, the try to poke you but will most likely end up orbiting you, try to keep them in the borders of the arena because Zwepel's dash attack will get rid of them. Bullet Nightmare is what you would expect: bullets come from the borders of the room but the twist is that one of them is a dream ice cream, which will give you a single piece of armor which will allow you to tank another attack before being reset.

Doomkins: Dooblion looking things will appear in the corners of the arena which will explode into rings of bullets if given enough time, if shot they will produce less bullets when they explode.

Defeating Zwepel rewards you with a candy-coated gumgum blaster.