Similarly, we say that those from Venus are Venusians and those from Mars are Martians, but we say we are from Earth, yet we refer to ourselves as “terrestrials” and not *Earthians*.

The first word anyone thinks of when thinking of a demonym for inhabitants of Earth is Earthling.

Other words used to describe us: Earther, Earthman, Terran, Tellurian, Gaian. I've even heard Earthan, though I think that was from a science-fiction character who wasn't sure what the right name was.

Also, before Venusian was popular in science-fiction, the word Venerean was common.

We also don't say moonian, we say lunarian, and in any science-fiction in which we say that, the inhabitants usually get ticked off if you call them lunatics. In The First Men in the Moon, they're called Selenites.

The choice of demonym isn't always place name + ian in English. Inhabitants of New York are New Yorkers. Inhabitants of Pittsburgh are Pittsburghers. Inhabitants of Vancouver are Vancouverites. Inhabitants of Vienna are Viennese.

Mars is kind of unusual in that it doesn't have a bunch of different names.

-- 
SuStel
http://trimboli.name