I don’t think any secret boss has been found yet. Only hilarious stuff which cannot be spoiled.

INTERACTION AND SPINEL DO NOT READ THIS. SPOILERS FOR UNDERTALE BELOW.

Glyde: after the ice-slide puzzle in snowdin, go down into the room where you find gyftrot. then go down the path to find a room with a long path leading upward and a door. then, walk around for 2 minutes. you should find glyde afterwards

Mad Mew-Mew: ONLY WORKS ON SWITCH. Go on the date with papyrus and interact with the giant sink. then after the date, enter the door below the sink and you should find mew mew.

So Sorry: during hotland, in-between elevators R-2 and L-2, there will be an invisible path to the north of the path. then, close undertale and change your clock to say october 8, 8:00. then enter the path and you should have the fight with so sorry.

Fabulous

In Chapter 3 and 4

oops, then i got nothing

there are secret bosses-

/j

it turns out theres an alternative ending
SPOILER WARNING:

theres a very hard boss with the roaring knight at the end of chapter 3
the alternative ending is if you can somehow beat him

note: the only clip i saw of someone beating him used hacks, so uh good luck

Oh gosh….

edit:(it needs to be beaten with fighting btw)

WHAT THE HELL HAPPENS I NEED TO KNOW.

i gotta sleep so take this:
https://m.youtube.com/watch?v=rttvJYeT4w4
goodnight