Justice League will be the fifth movie in the DC Extended Universe. When it airs, only a handful of the characters will have been properly introduced, with Batman, Superman, and Wonder Woman all having had full films. The rest of the league, The Flash, Aquaman, and Cyborg have only been seen so far in cameos, without their origins explained or personalities explored. Meeting them and learning who they are is going to be a big part of Justice League.
Not too much is known about Justice League yet, but a few things have been revealed. Including the villains. The League will be fighting Parademons, a race of very loyal if not too bright aliens. That loyalty is to their leader Darkseid, a ruthless alien leader who wants to rule the universe. The Parademons will be a vast enough threat to bring together the members of the Justice League for the first time in order to fight them.
And it looks like they might not be alone. According to CBM, some fans think they have seen yet another race join the fight. If you look closely at the battle footage in the recent trailer, you may see some Thanagarians.
In the above picture, to the right of the head of the leaping Amazonian woman, there is a bit of white, which some fans have believe could be the wing of a Thanagarian. At least two of these white-winged creatures can be seen in this brief shot (check out this gif to see them in motion), and the shot immediately preceding it shows a winged army on the battlefield. An alien race who can fly due to wings made of Nth metal, some of the more well-known known Thanagarians are Hawkman and Hawkgirl/Hawkwoman.
Hawkman and Hawkgirl have recently been seen in the Arrowverse, most notably in Legends of Tomorrow. However, in that series they were not Thanagarians, they were humans who were reborn every time they died. Their eternal resurrection was eventually attributed to interference from the Thanagarians near the end of the first season.
Thanagarians do exist in the DCEU, even though they have not yet been seen or mentioned in any of the films. They appear in the comic book prequel to Man of Steel, making them definitively a part of the universe. As can be seen in the released pictures and footage, the fight against the Parademons will not be a human only battle – the Amazons and Atlanteans are joining the fight as well – so it would make sense that the Thanagarians might also into the fray. It would appear that the human race is going to need all the help it can get.