I've been painting my old 6mm Space Marine EPIC recently and thought I'd draw some 40K monsters... it would be fun to illustrate the whole rule book but that sure would be a project. I ended up doing this lineup instead.
Beasts of Nurgle - These have a variety of models. In the lore they act like slobbering eager puppies, but they're also slugs so I'm not quite sure how they move.
Nurglette - The image of it with a hanging tongue just appeared in my mind. Probably my favoute concept on this sheet because it's very silly and Nurgle.
Disc Riders of Tzeentch - The SM-EPIC models of these might not be a good representation of the concept in earlier art and rules. It's unclear what the discs are, but they might be some sort of warp-dwelling disc-shaped sharks. For some reason they have gun barrels underneath.
Tyranid Attack Scout - I never liked the old Space Marine scouts with the landsknecht sleeves, but looking at my newer mid-late '90s metal scouts... I find them a bit boring and generic, whilst at least the landsknecht sleeves stand out a bit, and they might be appropriate for the sci-fantasy setting.
Samus - Loosely after the NES cover. Not sure if these are appropriate 28mm heroic proportions.
Daemonette - Juan Diaz style, but with the bent claws from the SM-EPIC illustration (which is very '80s punk in vibe).
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Also, I found a picture of the V (Visitors, 1983) laser/blaster gun and had to fool around with the details a bit, aligning and cleaning up. V was one of the first sci-fi shows I saw... I didn't own a VHS player back then so I only saw Star Wars in the late 80s iirc.