The following are four classic fantasy monsters explained away with cold, impersonal science (Scooby-Dooed with almost adequate explanations, really) and one weird monster I just sort of made up. But why?
A monster hunter campaign in a "fantasy" world that has no magic would be fun on the bun. Clerics are herbalist healers whose rituals accidentally work. Mages are shams and alchemists, but they totally know it. Oh those wily "wizards".
Alternatively, these could serve as a nice surprise/fuck-you to PCs expecting anti-magical protections and the like to work.
Or you could use this to drop a "fantasy" world into your scifi campaign.