mime (flux's fork)
fluxionary

adds a monster mimicking its surrounding nodes.

balrog (flux's fork)
fluxionary

adds balrogs.

Forgotten Monsters
DuckGo

Monsters and Bosses...

Mobs Ethereal Bosses
DuckGo

Add Bosses to "Ethereal NG" biomes!

Power Rangers Modpack
LC Creations

Adds power rangers and an API to create your own.

PaleoTest for Hades Revisited
SFENCE

Adds Not Only Prehistoric Fauna and Flora to Hades Revisited.

pilzmod
Phii

a spreading mushroom biome which you can spawn into the world, with its own monsters and a boss you need to slay to stop the biome's growth. Best played with bow & armor mod and after reading manual.