I prefer to believe that the Remans are a separate species from the Romulans, because one of the things I liked about
Nemesis (and yes, I like a lot about it, thank you) is that it finally showed the Romulan Star Empire as an actual
empire, i.e. a central state ruling over
other cultures, rather than the usual monoracial society that doesn't look or function like a real empire at all. A state that has no subject peoples has no business calling itself an empire. So giving the Romulans someone else to rule over actually made sense of the name for the first time in 36 years. Making the Remans just mutant Romulans takes that away and reverts them to the same old trope of an interstellar empire made up of only one species.
What is the preferred explanation for the prevalence of Romulan forehead ridges in the TNG era?
If you think about it, the vast majority of the Romulans we saw in TOS were wearing helmets that covered their ears and foreheads (so the productions wouldn't have to make up a bunch of background extras with pointed ears and eyebrows). The only 23rd-century Romulans we saw without helmets were the Commander and Centurion in "Balance of Terror," the Commander, Tal, and two technicians in "The Enterprise Incident," two commanders and a crewman in the animated series, Ambassador Caithlin Dar in TFF, and Ambassador Nanclus and several Romulan delegates in TUC. So I've long suspected that most Romulans had ridges all along, but in the 23rd century they were relegated to the lower classes, serving as lower-ranking soldiers and grunts while the more Vulcan-appearing minority held more elite roles like officers and diplomats. By the 24th century, that seems to have been inverted, since the only smooth-headed Romulans we've seen from that century were Nero and his crew, who were working-class miners.
I'm my head it goes down like this. Early earth explorers go to the Vulcanian's home world. They step out of their ship, "man this place is as hot as Vulcan." Or some other volcanic reference. From then on it becomes referenced by humans as Vulcan. The human's just keep using the term until it is also accepted by alien species, and finally the Vulcanians themselves.
In the early days of exploration there was probably some long distance stellar mapping. Some astronomer discovered a star with twin planets and decided to name them Romulus and Remus. That term came into common use probably during the Earth-Romulan Wars. It was used to name the native (though never seen) inhabitants of that world as Romulans.
That's the way it
should be, but unfortunately ENT: "Minefield" overtly established that the Romulans' name for themselves was pronounced like "Romulan." (In
Uncertain Logic I revised it into "Rom'ielln," which was as far as I could change it while staying consistent with Jolene Blalock's pronunciation of the word.) I'm pretty sure it's also been confirmed that the Vulcans' name for themselves sounds basically like "Vulcan" -- although
The Tears of Eridanus established that it was only one of several indigenous names for their planet, including T'Khasi, Minshara, Vulcanis, and Ti-Valka'ain.