With indie publishers beings the wacky scamps that they are, a bunch of them has released versions of many of prohibited pre-2E monsters with the serial numbers filed off.
I'm working on some comprehensive simulacrum D&D encounter tables and have been using these versions to fill in some gaps; so far this is what I've come up with:
Beholder - Eye of Terror from Labyrinth Lord Advanced Edition Companion
Bullywug - Batrachian from OSRIC; Thugtoad from the Swords & Wizardry Monster Book
Carrion Crawler - Carcass Scavenger from
Displacer Beast - Phase Tiger from
Githyanki - Astral Raider from Ancient Vaults and Eldritch Secrets (a pretty "loose" version compared to the ultra-detailed Fiend Folio entry but it's there)
Grell - The Beaktopus makes for a workable substitute...
Mind Flayer - Mind Eater from the Swords & Wizardry Monster Book; Lasher from Mutant Future; The Thelidu from the Swords & Wizardry Monster Book are pretty close to the Mind Flayer, but their hit dice are significantly lower
Slaad - Bat from Ancient Vaults & Eldritch Secrets has kindly assembled the Senufio; Claudio Pozas made the Ogdoad for 3E, but they're a bit far from the original for my liking.
The monsters that I'm unaware of having any OSR-ish analogues are:
Githzerai - I admit, they don't exactly set the imagination on fire
Kuo-Toa - A whole lot of fishmen & Deep Ones out there though
Yuan-Ti - A whole lot of snakemen out there as well, but I haven't come across anything that closely replicates the AD&D version
Are any of you folks aware of additional versions of the "product identity" monsters or non-open pre-2E monsters that I have missed?