Any creature not listed in the bestiary doesn't have their creature product listed there as well if the creature product is unique to them. For example, Willi Wasp is a boss not listed in the bestiary however his loot can also be seen in other creatures like Wasps, so it's not just unique to him. However, Abyssador is a boss and has an Abyssador's Lash
that's unique to him so it cannot be seen in the bestiary. I'll give some more examples of items not seen in the bestiary. I won't be able to list them all as this post has a limit-
Alptramun's Toothbrush 
Ancient Liche Bone 
Ancient Watch 
Astral Glyph 
Astral Source 
Bat Decoration 
Beast's Nightmare-Cushion 
Bloody Tears 
Bones of Zorvorax 
Brain Head's Left Hemisphere 
Brain Head's Right Hemisphere 
Brutus Bloodbeard's Hat 
Chitinous Mouth (Baron from Below) 
Claw of 'The Noxious Spawn'
Cruelty's Chest 
Cruelty's Claw 
Flames of the Percht Queen 
Flawless Ice Crystal 
Frozen Chain 
Frozen Claw 
Frozen Time 
Gnomevil's Hat 
Golden Bell 
Golden Cotton Reel 
Grimace 
Harpoon of a Giant Snail 
Huge Spiky Snail Shell 
Young Lich Worm 
Yirkas' Egg 
Writhing Brain 
Writhing Heart 
Withered Pauldrons 
Withered Scalp 
Vial of Hatred 
Sources-
Creature Products
List of Creatures Excluded from Bestiary