I have no doubt that plagiarism existed in the Greek and Roman antiquity: some authors must have copied material more or less directly from others without attribution. (The moral requirement to cite your sources is a later invention, as far as I know.) But are there attested ancient mentions of ancient plagiarism? For example, a comment like this would count as a mention: "This Marcus doesn't write his own books; he takes others' work and publishes it as his own."
I don't recall seeing any mentions to that effect, but I assume there are some. My first guess is to look at satirists, but this is hard to search. Something like this would fit the overall tone of Juvenal, for example, from what I remember.