Overlapping monsters (probably not exhaustive, as I type that from memory, and I'll not list unique monsters, namely fiendlords like Orcus, Geryon, Juiblex, and co.):
Blood Hawk (May be in Fiend Folio)
Dark Creeper/Dark Stalker/Skulk: a "dark skulker" is mentionned for the Fiend Folio, and it can as well be something new as just a mistake from the part of the "In the Work" articles and relate to one of these three critters.
Demonic Knight ((sort-of) Death Knight from the Monster Manual 2)
False Spider, Solifugid & Pedipalp (City of the Spider Queen Web Enhancement)
Giant, Forest ((sort-of) Giant, Wood from Monster Manual 2)
Giant, Sand ((sort-of) Giant, Sun from Monster Manual 2)
Groaning Spirit ((sort-of) Banshee from Monster Manual 2)
Lurker Above (City of the Spider Queen Web Enhancement)
Shadow Demon (Book of Vile Darkness)
Taer (Unapproachable East)
Troll, Ice (Unapproachable East)
Juju Zombie (Unapproachable East)
Yellow Musk Creeper (May be in Fiend Folio)
Yeti (Oriental Adventures)