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From the site I read, platies used to be called moons because of the shape of the spot near their tail, then the name changed, and I guess some people still call them that.
Nope before that I've never known that platies were called moons, but since your question pertained to the first link that popped up, I thought I might chime in
Yes, old timers call them moons because of the crescent moon shape before their tailfins. I remember my Grandpa saying he had some moons and wags at one point - never knew what he was talking about until many years later.