Q:

Which Green Lantern appears in Justice League?

In the 2017 film Justice League, the audience sees a flashback detailing how the races of the world banded together to defeat Steppenwolf of Apokolips. Amazons, Atlanteans, Gods, and even a Green Lantern all participate in the battle. The Green Lantern is promptly killed, and his ring floats away. Is the Green Lantern we see a pre-existing character from the comics, or was it created just for the movie?

A:

Yalan Gur (or a member of the same species)
The alien Green Lantern in the flashback resembles Yalan Gur, who first appeared in Green Lantern #19 (December, 1991), and who operated on Earth (and throughout Sector 2814) in the 10th Century.

(Image comparison: Justice League – Film Review & The Ultimate Case)
Note the cape, the enlarged mouth, and the red skin on the movie Lantern's legs and forearms.
A different angle shows that the film Green Lantern's teeth closely resemble Yalan Gur's teeth:

In the comics, Yalan Gur died in disgrace, having abused his power.  The Green Lantern in the film, however, died a heroic death.  It may be that this is the same character, with different personal histories in the two portrayals.  Another possibility is that these are different individuals of the same species.  Over time, the Green Lantern Corps (in the comics) has recruited more than one member(1) from some species.

(1) Examples:

Korugarian: Thaal Sinestro, Katma Tui, and the half-Korugarian Soranik Natu
Xudarian: Tomar-Re, Tomar-Tu, and Somar-Le
Human: almost too many to count

Long lifespans
Some of DC Comics' species — such as the Guardians and Larfleeze's people — live longer than planets, with natural lifespans measuring billions of years. According to Action Comics 1000 (2018), even humans from our time (such as Lois Lane) are alive and well five billion years from now, thanks to medicine.
Yalan Gur could be from another such long-lived species, or from one with a relatively short lifespan (say, a mere 100,000 years) that is nevertheless sufficient to encompass the 10th Century and the battle several tens of thousands of years earlier.

