Yeah, but every ant, rat, and snake is an earthling to. That’s saying we are from the planet earth. The other terms are more about being part of the political entity of earth. If you are a Marsling, you could immigrate to become an Earthican, but you can never be an Earthling. Same for the other direction, being from earth we may some day become Martians, but can never be Marslings. Source: it’s as made up as every other part of the English language.

Earthling if you’re an alien with bigger guns than us.

Terran if we got bigger guns than you.

Earther if you’re a racist alien.

Hooman if you’re an alien that wants to rip us off in trade.