If your looking for some alien monsters for any old school science fiction game you could do a lot worse than starting with the original Fiend Folio, I think. I'm not even talking about things like reskinning undead as nanotech animates or victims of exotic plagues (though you can certainly do that); I think there are a lot of creatures in there that are just straight up science fiction.
The first creature listed are aarokocra, which are just straight up birdmen--like the Skorr of the Star Trek Animated Series and a bunch of other places. The algoid is a psionic algae colony; the CIFAL a colonial insectoid intelligence (it even has an acronym name). Osquips are pretty much ulsios from ERB's Barsoom stories. The grell already looks like a pulp sci-fi monster: I think there was one in Prometheus, wasn't there?
Yeah, there it is.
Anyway, demon, devils, and elemental princes are out without substantial overall, but some less interesting monsters for fantasy purposes might be made a bit more interesting in a science fiction context. Lava children might be a silicon-based lifeform that (like the horta) needs to be contacted rather than killed. Yellow musk creepers and zombies (undead also-rans) would work great in a horror scenario on a deadly jungle world. Even the much maligned flumph is less silly when it's a weird alien (maybe).
14 hours ago