As well as the vanilla bosses, you can now fight new bosses in the game.

Below is a list of mods that contain custom bosses:-

https://www.curseforge.com/minecraft/mc-mods/mariums-soulslike-weaponry

https://www.curseforge.com/minecraft/mc-mods/the-graveyard-fabric

https://www.curseforge.com/minecraft/mc-mods/bosses-of-mass-destruction

https://www.curseforge.com/minecraft/mc-mods/spellblade-next