Snoke, technically, is derived from many sources, and the source that stands above the rest are cut ideas for Emperor Palpatine, namely that he's tall, is humanoid but not human, looks exceedingly old but not decrepit, has bright blue eyes, wears a gold robe, mostly just communicated through holograms, was a puppet of somebody else, and is a Master of the Dark Side of the Force, but unlike Darth Vader, probably didn't identify with any one movement. But that can't be the only source, not at this point. So what other sources are there? Well... I think that first of all, it's clear that Snoke wasn't always meant to be Force-grown, because he was said to be "handsome in his youth" and "a couple of hundred of years old", unless Andy Serkis was talking about Darth Sidious, which are both tremendously easy to believe, given what young Ian McDiarmid looked like and that Force-sensitives living tremendously long lifespans is the most normal thing in the world, but otherwise, I would think that some of the plans for Snoke changed over the years, and the real reason why he knew so much about the Second Death Star events would boil down to either the fact that like Yoda, Snoke could see things happening from far away, or that like Darth Vader, he could pluck the knowledge from Luke's head. But we'll start by focusing on the Snoke that we got, shall we? A very close equivalent to the Snoke as a whole, both what we ended up getting and what we were likely supposed to get, would be Atha Prime, a character created by Kenner during the 1980s to continue the Star Wars toy lines. Though he's technically not a Legends Character unless we're talking about Zeta Magnus, if we were to combine everything we know about Atha Prime with that of Zeta Magnus and Lord Shadowspawn, we pretty much get Snoke. Atha Prime himself was described as a "Ruler of the Dark Worlds" like how Snoke ruled the Unknown Regions. He was an arbiter of the Clone Wars, and though he was exiled for unknown reasons (remember, back in those days, the Clone Wars was most probably envisioned as a Jedi rebellion defending senators who were against Palpatine against Shocktrooper cloned warriors, meaning that it's likely that Atha Prime wasn't exiled for opposing Palpatine but rather, Palpatine gave Prime's clones a "Night of the Long Knives"), he emerged from exile and apparently ended up helping Imperial Loyalists after the Emperor's death, who apparently made him a mobile command post in the form of a gigantic Star Destroyer Atha Prime's status as an arbiter of the Clone Wars, genetics terrorists, and grower of clones, likely implied that he himself was a clone, something that Zeta Magnus serves to confirm. Zeta Magnus was the closest thing to a canonical Atha Prime we got, and he was genetically engineered by the Arkanian Dominion's scientists to be a super-soldier. While Snoke doesn't really resemble any one kind of Star Wars species, he does most closely resemble a weird kind of Arkanian offshoot. The Arkanian Offshoots, unlike baseline Arkanians, were often almost devoid of melanin, five-fingered, and tall. Also, remember that Arkanians came in even more varieties than we really saw in Legends, so there's a chance that there's some kind of race of Arkanians out there that looks like Snoke. But Zeta Magnus was not the first attempt to make Atha Prime canon. An earlier attempt was to consolidate him with Lord Shadowspawn, and Snoke has some particular overlaps with Lord Shadowspawn as well. Shadowspawn was what Lord Cronal called his puppets, namely Nick Rostu wearing the Shadow Crown for Cronal's control. Cronal needed that puppet because he was weak, and was planning on using Shadowspawn as a tool to take over Luke Skywalker's body. Likewise, Supreme Leader Snoke was the puppet of Darth Sidious, who was physically weak and planning to possess the body of a Jedi. But that's covering Atha Prime. There are more characters whom Snoke bears semblance to, such as Joruus C'baoth. Joruus was the rogue clone of a Jedi Master from long ago, and he was likely meant to be the same character as the Guardian, or the Guardian was another Jorus clone. Whatever the case, the Guardian's task was to watch over Palpatine's stuff in Wayland, but Joruus, whether for reality or in his insanity, seemed to believe he killed him. Like Snoke, he heavily specialized in telekinesis and telepathy, knew Luke personally and seemed to support Luke's Jedi endeavors, and sought the son of Han and Leia to train to his ideology, which asserted that Force-sensitives had a right to reign over those who were not, which Supreme Leader Snoke also seemed to believe. Then we have Supreme Prophet Kadann. The character who operated under this name was many things, namely because there were two of them. The fake, Rajah Ubooki, was prone to using tricks to make himself seem more imposing, such as Snoke does with his blood-red throne room, and he was actually working as a puppet for, again, Cronal, to keep the warlords divided and the masses united before he went out of control and tried to take over the Imperial Remnant for himself. As for the real Kadann, he was a Jedi who turned to the Dark Side, seeing value in both sides of the Force. Like Snoke's depiction in The Force Awakens, he was soft-spoken, and was not prone to displays of power, before the personality retcon in The Last Jedi. And we have the Sith Emeperor, who had also specialized heavily in telepathy to the point in which he was able to sculpt Revan to fit his desires, and he had many puppets, his essence partially divided across them. Then we have Lumiya, who was the Sith apprentice of Darth Vader, who trained Jacen Solo to become Darth Caedus, just as Snoke trained Ben Solo to become Kylo Ren. And of course, it's similar to how Shimraa Jamaane, the Supreme Overlord of the Yuuzhan Vong, was the puppet of the sadistic and powerful Vong Shaper, Onimi. Snoke seems to have been many things in pre-production: Possibly an alien possessed by Palpatine but not a clone (which would explain his knowledge of the DS2 events, though that could easily be explained by how omniscient a lot of Force users are), possibly a warlord and guru from the Unknown Regions who helped the Imperial Remnant (which would fit with hints in the books, but the hints could have meant anything), possibly a former Imperial Inquisitor and ex-Jedi who resented both the Republic and Empire (which would suit the Mengele imagery, but that could have been meant to fit with the idea that the First Order is like Nazis hiding in South America), and possibly even the former Darth Plagueis (which would explain why the arrangement was like the opera's and why the melody was similar to the melody that played during Palpatine's story, but that could have been a Palpatine hint too). The only point was that Snoke was the "uber-villain" who would be indirectly responsible for much of the conflict, so tons of ideas were seemingly placed on the table. The very first of these seems to be nicknamed "Uber" as a "working title". "Uber" was apparently an ancient Sith ruler who was locked in a tomb long ago, one that could only be unlocked with the crystal in Luke's lightsaber. But his mind still worked, and from afar, had been affecting various darksiders, motivating Talon and later others to do whatever it took to find him and release him.. With this in mind, Uber's design seems to come from Marka Ragnos, and his role in the story seems to come from both Ragnos and Exar Kun. Ragnos was an ancient Sith leader whose spirit persisted in a tomb for a long time, and he was being recovered by the Imperial Remnant, Exar Kun's spirit was trapped in a tomb in Luke's Jedi Temple on Yavin IV, and he communicates with Kyp Durron, a pupil of Luke's who Kun was turning to evil. So these are the Legends roots of Snoke. Let me know in the comments which ones do you think fit Snoke the most, or which ones would have been the most interesting, or which ones do you think they were shooting for to begin with!