[dusts off "Volume 2 -- Monsters & Treasure"]
Hmmm..
Never used gorgons (as in big brass bulls) or manticoras or chimeras or sea monsters or nixies or ochre jellies...
[now more sensibly moves onto MM]
Never used...
Aboleth, Achaierai, Allip, Ankheg, Aranea, Arrowhawk, Athach, Azer, Behir, Beholder, Belker, Blink Dog, Bodak, Bulette, Celestials, Chaos Beast, Chimera, Choker, Chuul, Couatl, Darkmantle, Delver, Demons, Destrachan, Devils, Devourer, Digester, Displacer Beast, Dragonne, Drider, Ethereal Filcher, Ethereal Marauder, Formians, Forst Worm (never had a game in an appropriate enviroment, but would use it), Fungi, Gibbering Mouther, Girallon, Gorgon (...still...), Grey Render, Grick, Grimlock, Howler, Krenshar, Kuo-toa, Lamia (utterly different vision of what a Lamia is...), Lillend, Locathah, Magnim, Manticore (but might someday), Mephits, Mimic, Mind Flayer, Mohrg, Oozes, Otyugh, Owlbear, Phantom Fungus, Phase Spider, Phasm, Planetouched, Pseudodragon, Rakshasa, Rast, Ravid, Remorhaz, Roper, Rust Monster, Sahuagin, Sea Lion (but might someday), Shadow Mastiff, Shield Guardian, Shocker Lizard, Skum, Slaad (of any variety), Spider Eater, Sprites, Tarrasque, Tendriculous, Thoqqua, Tojanida, Umber Hulk, Vampire Spawn, Vargouille, Winter Wolf, Xill, Xorn, Yeth Hound, Yrthak, Yuan-Ti
Monster Manuals are a terrible investment for me...
OTOH, I have made up dozens of setting-appropriate (i.e. to my own campaigns) creatures and used several from the Creature Collections.
Overall, I find many of the monsters ... well, silly...