The singular is "protozoon". It's a New (scientific) Greek neuter; -on in the singular, -a in the plural.
Gk. spelling: prwtozwon.
That’s a singular that I doubt will gain popularity with use, as it just sounds (literally) silly, and if you used it, you would have to spend several posts explaining why, and they still wouldn’t believe you.
It does kind of roll off the tongue, however. I’m thinking of some space alien. “I am Protozoon of the planet Fnargle!”