All the monsters since the start of the thread:
Tasloi: Oriental Adventures
Giant Space Hamster: Tome of Horrors
Witchlight Marauder: --
Leucrotta: Monsters of Faerûn
Piercer: Tome of Horrors (as a 3.0 creature), Tome of Horrors 2 (as a 3.5 hazard)
Meenlock: Monster Manual 2
Flail Snail: Tome of Horrors
Quillion: --
Carbuncle: Tome of Horrors
Flumph: Tome of Horrors
Aleax: Book of Exalted Deeds
Magnesium Spirit: Tome of Horrors
Giant Lynx: --
Cave Ape: -- (but could a dire ape fit the same role)
Thoul: --
Living Statue: --
Giant Otter: --
Pernicon: --
Stained Glass Golem: Monster Manual 2
Scarecrow Golem: There's a Scarecrow construct in the Tome of Horrors, but IIRC it's not a golem (no immunity to everything but yellow spoons and Nystul's magic aura, for example)
Let me add mine:
Beholderkin (Director, Examiner, Hive Mother, Lensman, Observer, Orbus, Overseer, Searechter)
Dabu
Demon (Bulezau, Molydeus)
Fundamental (Air, Earth, Fire, Water)
Rilmani (Abiorach, Argenach, Plumach)
Spellhaunt