Forgotten Monsters
DuckGo

Monsters and Bosses...

Mobs Ethereal Bosses
DuckGo

Add Bosses to "Ethereal NG" biomes!

Alive AI
AiTechEye

Advanced survivor ai mobs (npc, monsters and more)

balrog (flux's fork)
fluxionary

adds balrogs.

Voxel Dungeon
Noodlemire

This is a recreation and adaptation of an Android rogue-like game, Pixel Dungeon by Watabou.

Power Rangers Modpack
LC Creations

Adds power rangers and an API to create your own.

Grave
ElCeejo

Adds an eerie biome with multiple mobs.

Native Villages
Liil

Adds villages of native people to the world.

Nether Mobs
rael5

Add monsters and dragons to your nether world.