Justice League ended up being a...well, mess is the polite way to put it. From a cinematic perspective, the film is a bit of a Frankenstein, with Zack Snyder and Joss Whedon's reshoots taking away the effectiveness of both. Thing is, the film isn't solely a mess cinematically, as it's also a mess behind the scenes, and that is definitely the case regarding original Ares actor Nick McKinless. McKinless is the actor Snyder originally cast in the role of Ares, and you can even see him in the finished version of Justice League, though you won't find him in the credits as the character, and he is missing some checks because of it (via ScreenRant).
Ares appears in the movie during a flashback scene that shows Steppenwolf attempting to take over Earth, stopped only by the combined forces of man, Atlantis, and the Gods. That group included Ares, who ends up going toe to toe with Steppenwolf in the film, but that encounter is brief. It turns out in the original cut of the film (the famed Snyder Cut), McKinless had an extended fight with Steppenwolf, though in that sequence it wasn't actually Steppenwolf he was fighting, but Darkseid.
In the original fight, Ares lunges at Darkseid and knocks him to the ground with his axe according to McKinless, and then swings the axe around to presumably knock away some Parademons. In the actual fight, McKinless was fighting a green clothed stand-in, as a stunt performer was going to provide motion capture separately.
McKinless said he was specifically contracted by Snyder to play Ares, and he also explained the early process of coming up with their take on the character.
“Zack, Eunice Huthart (stunt coordinator) and myself sat in Zack’s office and discussed the scene and what look he wanted extensively and he clearly stated I would be Ares," McKinless said. "It was that way right up until DC stepped in to completely change the make-up and costume after Zack had pretty much signed off on our original ideas. We did so many changes it was getting ridiculous frankly… Zack’s vision [was] skin like paper and veins like worms. I managed to implement a plan and train myself to get the required results. As well as being a stunt coordinator, I’ve also trained athletes and actors my whole life so I knew what was necessary”1comments
You can see McKinless in the final film, though the sequence is cut short, and in a later shot, the part seems to be played by David Thewlis, who played Ares in Wonder Woman. Despite being a more integral part of the Ares role in Justice League though, McKinless is not credited as Ares. He doesn't even share co-credit, instead credited with Stunts. He also hasn't received any residuals from Justice League either, despite his other Greek God co-stars Sergi Constance and Aurore Lauzeral receiving both credit and residuals.
McKinless has tried to reach out to Warner Bros but hasn't received a response, and we're not sure why he ultimately ended up not getting credit. This movie was quite the mess from every angle, and we hope McKinless gets an answer sooner than later.