Be warned, though. The quality of these mods may vary. But they are monster packs nonetheless -- and they deserve to be shown to everyone to try out.
DoomRL: Monster Pack
Dark Doom Creatures
Hard Doom
Colorful Hell
Colorful Hell Custom Difficulty Addon
Kriegsland Enemy Pack
Bratwurst
The Western Patch (ModDB Link)
Legion of Bones
Stylish Doom (Good for La Tailor Girl!)
Doomless
Shades of Doom
CHAMPIONS
Cyber's Monster Randomizer
Untitled's Touhou Doom Gameplay Mods
DoomKraken's Monster Randomizer
Legendoom
Rampancy
Combined Arms
Vanilla Doom Plus - Monster Randomizer
Less Frustrating Doom Monsters!
SpaceHunter Enemies Standalone
ContraDoom Monsters
Daggerfall Monsters
Catacomb 3D Monsters (ModDB Link)
Starship Troopers Monsters (ModDB Link)
Left 4 Doom Monsters (Recommended to use AI Director!)
ScoreDoom Monster Replacement Randomizer (ModDB Link)
Project Malice (Discord Link)
Rebel Rumble Monsters-Only
Mellow Heretic
Sun Damage Omen Enemies Pack
Angelic Aviary | (Doomworld Link, Formerly known as "Angelic Waifus")
KRUGLY! (Cacodemon Mayhem!)
Standalone DOOSK Monsters (ModDB Link)
Meatgrinder Monsters Only (ModDB Link)
Smooth Doom Monsters Only (ModDB Link)
SMart Extreme UNhindered MONsters (SM-EX-UN-MON) (ModDB Link)
Theatrical Horror of Hell's Terror Monster Pack (ModDB Link)
AvP Monster Pack
Shut Up and Bleed Monsters (ModDB Link)
Dehacked Attack - Blast from the Past
Heretic: Legions
Brutal Doom: Monsters Only (ModDB Link)
Realms of the Haunting: Monsters Only (ModDB Link)
Terminator Mayhem: Monster (ModDB Link)
Xenomorph Madness
Rebel Rumble Monsters Only
Xim's GZDoom3 (Monsters Inside!)
Russian Mercenaries
ERLBA Monster Pack (Hexen)
EVP Monsters
Xim's Star Wars Doom Monsters (ModDB Link)
Scalliano's Shuffle GOLD (Monsters Inside!)
LEGION
Wave Attack! (In OP)
DoomLess SNAFU Cut
Universal Entropy
10.5x (DOOM.WAD, but it's Hell Revealed 2)
Corruption Cards
Duke Nukem 3D v10b Monsters (ModDB Link)
Classic Doom3 Monsters (ModDB Link)
Honorable Mentions:
Babel
Monster Enhanced AI
If anyone has any more suggestions with links, please hit me up.
Spoiler: Original Post