9. Two-Mouthed Wall - Cuphead (2017)
What's definitely NOT unexpected about Cuphead is its ludicrous difficulty, whether you're playing solo or with a friend. There are an insane amount of tough bosses to fight your way through over the course of the game, as well as a fair few sub-bosses that show up to give you a bit of pre-boss aggro.
In the level, Funhouse Frazzle, you'll come across one such sub-boss, the Two-Mouthed Wall. Literally a wall with a mouth on both the floor and the ceiling, this enemy spits out red lip projectiles and lines of toy cars at you until you deal enough damage to its one central eye.
Later on in the level, you'll find another identical wall blocking your path once again. This time, however, the toy cars are replaced with a long tongue that can whip out towards you with only a split-second warning.
Now, it's clear that these are two separate enemies, so the latter's different attack isn't technically a 'second phase' of the former. But, seeing as no other sub-bosses appear more than once in the game, much less in the same level, the close proximity of these two encounters can easily be seen as a two-stage boss battle with a small break in-between.