There are multiple choices and themes present in Homes, Moldvay, Cook and LL for monsters of the past. The are plenty of "Lost World" monsters (dinosaurs, neanderthals, saber tooth cats, etc.) as well as ancient Greek monsters (minotaurs, cyclops, medusa etc.). The challenge I am facing is finding monsters to effectively represent the future.
What follows is a preliminary list of monsters from Homes, Moldvay, Cook and LL (those not included in Holmes and Cook will appear in [brackets]) and an explanation as to why they suggest a future timeframe to me:
- [Ape, Albino] and [Baboon, Higher] — Four words: Plant of the Apes.
- Black Pudding — I have always felt that black pudding would fit better on an episode of Star Trek rather than as a stand-in for some primordial slime.
- Blink Dog and Displacer Beast [Phase Tiger] — These two have a definite sci-fi flavor. Their mutual hatred suggests a level of intelligence not necessarily represented in their statistics. I image the displacer beasts to be the equivalent of space-born Nazi war criminals and the blink dogs to be akin to a space-faring version of the Mossad hunting them down. The only question is, are the blink dogs actual agents or are they the pack of hunting dogs used by the agents (which would be some other kind of alien)?
- Carrion Crawler [Carcass Scavenger] — This tentacled monstrosity has more than a little HPL-inspired flavor. While I do use HPL as inspiration a lot, I usually do so from a sci-fi/futuristic POV rather than a fantasy one.
- Doppleganger — I despised DS9, but Odo proved the doppleganger is just as appropriate for a sci-fi setting as it is for a fantasy setting (if not moreso).
- Gelatinous Cube — I have an easier time believing this to be the result of a science experiment gone wrong than the consequence some kind of magical research.
- Ghoul, [Ghast] — These undead most closely resemble the zombies found in all those zombie apocalypse movies cranked out by Hollywood every couple of years.
- Golem, Bronze — This could very easily be re-skinned as a robot.
- Gorgon — Likewise, this could easily pass as some kind of living machine.
- Men, Berserker — Reavers. Serenity. Nuff Said.
- [Morlock] — as an homage to the inhabitants of H.G. Wells' dystopian future from The Time Machine, this is one of the few (if not only) D&D monster that is explicitly from the future.
- Ochre Jelly — Described as a giant amoeba, I can't help but think of the Invisible Monster from the episode of the same name from the first season of Johnny Quest. (This could also be the inspiration for the Invisible Stalker; however, I like Ochre Jellies better)
- Owl Bear — although described as the result of a magical experiment, this could also easily pass for a mutant from the future or the result of a genetic experiment.
- Rust Monster — These guys just beg to be seen as biological constructs created to clean up abandoned cities after some kind of future war that levels most, if not all, urban centers.
- [Statue, Animate (Iron)] — Like the Bronze Golem above, this can easily be re-skinned as a robot.
- Zombie — While ghouls and ghasts do more accurately depict Hollywood's version of zombies, that doesn't mean I can't also have the D&D version, too.