Warning: Contains SPOILERS for The Suicide Squad.
Theories abound regarding the identity of Peacemaker's villain, but there are numerous possibilities from DC Comics as to who The Suicide Squad spinoff's antagonist could be. Little is known about the menace that apparently inspired Amanda Waller to order that Peacemaker be rescued and recruited to a new covert team in the spinoff. However, there is ample reason to believe that a mystery villain will be revealed over the course of the series, much like how Starro the Conqueror's presence in The Suicide Squad was a closely guarded secret until the final trailer.
The only solid fact that about the villains in Peacemaker is that they are known as Butterflies. This is not a name connected to any previous DC Comics team or villain and it seems unlikely that Peacemaker might be used to launch a crossover with The Venture Bros by introducing a live-action version of the Mighty Monarch. It is more probable that the Butterflies are an original creation of writer/director James Gunn, who previously created a number of superhero and supervillain characters for his original films Super and The Specials. However, Gunn has also proven his knowledge of obscure DC Comics characters, modifying a number of them for inclusion in The Suicide Squad, prompting theories of a secret villain.
The trailers for Peacemaker have shown him fighting a green-costumed villain, who is believed to be a character called Judomaster. This is odd, as the various versions of Judomaster in the comics were all heroes and wore a costume that was predominantly yellow and red. Of course, fights between heroes who mistake one another for villains are quite common in superhero comics, as is the practice of heroes disguising themselves in new costumes and posing as villains so they can get the drop on the bad guys. In any case, there are a number of DC Comics villains who might make interesting adversaries for Peacemaker in future seasons, even if they are not the mastermind behind Peacemaker season 1. Here's every possible theory as to who it could be.
Killer Moth
First appearing in Batman #63 (February 1951) Killer Moth's gimmick was as idiotic as his name, marking him as one of DC Comics' lamest villains after Kite Man and Condiment King. Reasoning that the lawbreakers of Gotham City needed a costumed protector as much as the decent citizens, Killer Moth offered ordinary criminals protection from Batman–for a price. Despite a special "cocoon gun" that let him bind his targets in sticky silk like Spider-Man's webbing, Killer Moth was easily defeated by the Dark Knight on multiple occasions.
Killer Moth's main claim to fame (or shame) is that he was the first supervillain defeated by a then-untrained Barbara Gordon. She thwarted Killer Moth's attempt to kidnap Bruce Wayne while on her way to a party in a Batgirl costume in her original origin story as Batgirl. The incident inspired her to take up crimefighting in earnest. While there's no direct evidence to indicate that Killer Moth will be making an appearance in Peacemaker, it does make sense that only a villain with the poor judgment to take on Batman while dressing like a neon-colored moth might also lead a terrorist group called the Butterflies.
Captain Nazi
First appearing in Master Comics #21 in December 1941, Captain Nazi was originally created by Fawcett Comics as an enemy of Captain Marvel (now Shazam) and Bulletman. Born Albrecht Krieger, Captain Nazi was given enhanced strength, speed, and endurance by a formula developed by his scientist father. Best known today for an origin story that is reminiscent of Steve Rogers' transformation into Captain America, it would be interesting to pit Captain Nazi against Peacemaker, whose character has been summarized by both actor John Cena and writer/director James Gunn as a douchebag version of Captain America.
The Controller
Aloysius Clarke was a scientist who, like many scientists in the DC Comics universe, unwisely worked for a crime syndicate before being betrayed by his employers and left for dead. However, while his body was left paralytic, Clarke's mind remained unhindered, and a year in traction inspired him to ponder new applications for cybernetics. Much like the Daleks' creator Davros in Doctor Who, Clarke designed an armored life support chair which restored his mobility, as well as an army of deadly robots he dubbed Executioners.
Now calling himself the Controller, Clarke made a play for the control of New York City's gangs and crossed paths with Adrian Chase (aka Vigilante) on more than one occasion. The Controller ultimately died a slow death after Vigilante cut the power to his life-support chair. With a different version of Adrian Chase acting as Peacemaker's partner in crime-fighting in Peacemaker, it would be fitting for the series to present a new version of the Controller to act as the big-bad behind the Butterflies.
Kobra
Abducted by a snake-worshipping cult as a baby, Jeffrey Burr was believed to be the cult's Chosen One whose rise to power would destroy the corruption of the modern world and usher in a new era of enlightenment and peace. As Kobra, Burr built the cult that raised him into a criminal empire that rivaled the League of Assassins. Impressively, Batman once described Kobra as second only to Ra's Al Ghul among his enemies in terms of how potentially dangerous he was.
While he often employed armies of faceless drones in green costumes similar to the one seen in the Peacemaker trailer, Kobra was not above recruiting freelance supervillains to act as his catspaws. He once established a special Strike Force Kobra made up of villains like Synoide, Lady Clayface, and Windfall to take on Batman and the Outsiders. Given that, there's a chance that he could be the power behind the Butterflies in Peacemaker, despite the name not fitting his reptilian theme.
TDK
Introduced in The Suicide Squad as a modern-day precursor to the Legion of Superheroes reject Arm-Fall-Off-Boy, Nathan Fillion's The Detachable Kid (TDK) was one of the many squad members who became cannon fodder in the movie's bloody opening sequence. However, TDK was still alive during his last scene in the movie and James Gunn later confirmed that TDK somehow survived the events of the film. Given that, and Gunn's long history of always offering his good friend Nathan Fillion a cameo in his projects whenever possible, it's a plausible scenario that TDK might return in Peacemaker, seeking revenge on the hero who was saved and brought home when he was not.
