This reminds me of The Peaceable Kingdom (1834), a painting by 19th-century American painter Edward Hicks. He painted over 60 versions of the same scene over a span of several decades. Without unpacking all of the symbolism, you'll notice the abundance of odd-looking, large felines. Despite the fact that it was painted less than 200 years ago by a modern artist, he had never seen a lion, so he painted large house cats.
Despite living around Philadelphia in the early 19th-century with all of its intellectual resources, despite living on a continent filled with other large felines, despite a huge number of historical advantages afforded to this modern figure, it was STILL an EASY mistake for an American painter to simply not have known what a lion looked like.
While it is the case that "we would deduce that MANY people, even if not 'historical figures,' were familiar with [lions] in an unbroken stream throughout history" it just so happened that right in the middle of a time period FULL of people who were familiar with lions, here's an famous painter in Philadelphia in the 19th-century who was so utterly unfamiliar with lions that he painted oversized house cats.
Generalized deduction is not enough. It's just circumstantial.
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Since this entire discussion is predicated on a physical piece of art, we are burdened with a necessity of finding more evidence in the form of other artifacts – rings, coins, stone inscriptions, busts, or portraits – to which Raphael would have had access.
Given the bearded figure's similarity to Epicurus in the fresco (to the immediate left of Plato), it seems highly likely that Raphael was familiar with Epicurus' bust, and transferred the face of that bust to this figure ... so it seems, anyway.
To which ring, coin, inscription, bust, portrait, or description did Raphael have access?
Suppose the possibility that Raphael wasn't intentionally referencing Epicurus; he was just re-producing the unlabelled bust of a Greek he saw. Not all of the figures are named. Raphael was a 25-year-old artist who did NOT spend his youth studying comparative Hellenistic philosophy. He was being paid by the Vatican Church to glorify the Eternal Divinity of Christ. This was a commissioned, Vatican project in the Apostolic palace, not Raphael's version of the Beatles cover of Sgt. Pepper's Lonely Hearts Club Band, full of his favorite philosophers. Epicurus could have been featured accidentally.
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Even still, we're playing "Where's Waldo?" with a Renaissance fresco. This painting was not created so bishops could play "match-the-face-with-the-name" in the Apostolic Palace. It was created to glorify Jesus Christ and His Church.
Epicurus doesn't need to be in the painting; it doesn't support the overall message of the commission. Placing him there would have been a subversive choice of the artist. That's a hell of a bold statement for 1508. It was a bold statement for Sinéad O'Connor to have ripped the Pope's picture on Saturday Night Live in 1992. Like I said, it's like Rivera painting Lenin in the Rockefeller building. It begs more questions.
The Church has a history of destroying artwork (and artists) that didn't support their narrative, so why would Epicurus have been allowed on a wall in the Apostolic Palace?