Martian

Martian refers to anything that pertains to the planet Mars.

Doom 3
In Doom 3, the Martians were an ancient race of intelligent beings that were nearly destroyed by the forces of Hell. As the game progresses, the player learns that archaeological studies were conducted on Mars, leading to the discovery of this civilization. One important note is that the Martians created the Soul Cube as a means to defeat the Hellish invasion.

Most of these Ancient Martians died during the demonic invasion or were sacrificed to create the Soul Cube. The few survivors then abandoned Mars and what ultimately became of them is unknown. There are theories that they fled to Earth and that humans may in fact be their descendants.

Doom RPG
When the Marine reaches the Reactor there is a computer that contacts him and speaks in Martian, "Risi Soru kol feri Asi'ril.", translated means "Red Planet finds you worthy and offers to help." It continues "Ned'ai Risi Soru.Fele Khamik Nari felil sani." which translates: "Red Planet trembles. The way is made clear."

Doom 2016
In the first mission The UAC after the player leaves the elevator, and a climbs a few edges, the inital outside arena appears to show ancient structures resembling those of the Martian structures of Doom 3, even so far to have been excavated and glowing hieroglyphics on the central pillar, whether these are Martian in origin, or Demon is unknown.

Interesting enough a Soul Cube appears in the Lazarus Labs, whether this is a sign of things to come, or simply an easter egg is unknown, although nevertheless it can be considered to be connected to the prior ruins from before.

Trivia

 * Whilst the Martians are never physically shown, several murals, and ancient metallic carvings showing them, along with the Facial structure on the Soul Cube, shows a resemblance to Gray aliens, a common trope in Science Fiction media.