The idea of reptilian humanoids is one that's quite common in science fiction, but rather less so in fantasy literature, at least in its early days, presumably because they don't have much in the way of mythical antecedents. H.P. Lovecraft referred to a reptilian race in passing in his 1921 short story 'The Nameless City', while Robert E. Howard introduced a race of serpent people as antagonists in his own writings starting in 1929. Since Howard and Lovecraft were friends, later works considered these two instances to represent the same race, and they became part of the Cthulhu Mythos, and, eventually, the Call of Cthulhu RPG.
Although beings with a mix of humanoid and reptilian features do exist in some mythologies, none have much resemblance to lizardfolk as they exist in D&D, and it seems plausible that Gygax had something like Howard's race in mind when he created them. (Having said which, reptilian humanoids did also exist in SF at the time; Doctor Who's Silurians pre-date D&D, for example, but seem an implausible inspiration).
