Not sure . . . But at one time germs and viruses were thought to be demons. Granted, they weren't microscopic demons, but a long time ago, when people became ill it was thought to be demon oppression or even possession.
So, in a way, what was thought to be myth, was really microscopic entities.
Well ... "How many angels can dance on the head of a pin" is a philosophical question. So maybe angels.
Well, not exactly mythological ... But certainly whimsical: A Who
Mythological beings have a tendency not to be protists. Bigger creatures are preferred.
Interdementional and a live.
Not mythological.
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