Star Wars: The Rise of Skywalker has promised to finally feature the Knights of Ren, militant followers of Kylo Ren and the First Order, but sources refuse to give many concrete details about them - including their species. The appearance, age, and gender of the dark side warriors remains a complete mystery, feeding into the hype surrounding these long-awaited villains.
The Knights of Ren have been seen before, of course - in Star Wars: The Force Awakens, appearing in the visions Rey experiences from touching Luke Skywalker's lightsaber on Takadona. In the vision, Rey witnesses the Knights standing behind their leader, Kylo, as he cuts down an enemy warrior. That brief glimpse was the first and last audiences knowingly got, though some theorized (incorrectly) that the Praetorian Guard protecting Supreme Leader Snoke in The Last Jedi were in fact the Knights of Ren. Additional sources like The Force Awakens: The Visual Dictionary confirm some basic information, but it's been frustratingly unclear since then who exactly the Knights even are.
A new reference guide expanding on the mysterious group, Ultimate Star Wars New Edition, has continued this trend and refuses to name their species. The small excerpt it devotes to the Knights of Ren reads:
"The Knights of Ren are Kylo Ren's most deadly and mysterious servants. With their bodies encased in rusty, battered armor and their faces permanently concealed beneath ominous masks - echoing that of the new Supreme Leader himself - even their species remains a mystery. What is obvious to anyone who witnesses them in action is that their fighting skills and martial prowess are without equal. Each Knight is armed with a lethally effective and unique weapon, suited to either long range or close-quarter combat."
What we do know about the Knights of Ren is, as you might expect, pretty limited. We know that they are absolutely not Sith, nor are they tied to the legacy of Darth Vader in any way. We know they do not serve the First Order, but rather exclusively serve Kylo Ren and aid the Order as a consequence. We don't know if any or all of them can use the Force - sources on that question are especially vague, and have implied an answer in both directions. And, of course, we know that their role in Episode IX will be much more substantial than their brief appearances and mentions in the past.
Any information at all about The Rise of Skywalker has been kept close to the chest by director J.J. Abrams and Lucasfilm. The whole film is arguably even more secretive than The Force Awakens was four years ago. Fans have launched countless wild, speculative theories about who the Knights of Ren might be and how they could play into the final film of the Skywalker Saga. Some have claimed that they may end up related to the newly-revealed Sith Troopers - red-armored Stormtroopers whose name implies some connection to the returning Emperor Palpatine. Others wonder if the Knights of Ren are the former Jedi students of Luke Skywalker, following Kylo to the dark side. And there's even a theory they are Force-sensitive Mandalorians, which would allow the movie to perhaps tie into the upcoming TV series, The Mandalorian. Whatever the truth of these shadowy warriors, we obviously aren't going to be told much before Episode IX arrives.
Source: Ultimate Star Wars New Edition