What does Moonclan Darkclan and Cloudclan have to do with Skyclan?

I didn't know that Moonclan, Darkclan, And Cloudclan were even real clans.

'''Maybe they're Fan Clans... because SkyClan used to live with ThunderClan, ShadowClan, WindClan, and RiverClan until their territory got destroyed by twolegs and they left. I think Heathertail is leader of Darkclan.'''