Whether or not Aesop, an actual person, reported to be a non-Greek slave on the island of Samos, actually existed is something of an unanswerable and unknowable question, but if you’re interested in the greater Aesopic tradition that doesn’t necessarily mean the end of the road.

Aesop as a sort of culture hero continues to be written about throughout Antiquity. Aesop appears as a figure in both visual and literary culture in Athens by the fifth century BCE, by which time there were already parts of an established tradition about him that different authors draw from in representation. Greece already had a fable tradition, appearing most often in Archaic lyric poetry, by the 6th century BCE, when Aesop supposedly lived (the existing, orally-circulating fable tradition may be why you’ve heard that Aesop was many people and not just the one, since this larger tradition was not the work of a single person), and it’s in the fifth century BCE that Aesop begins taking credit for these fables. An oral tradition also existed in Antiquity with stories of Aesop’s life, some of which were written down into Lives. These written biographies span quite a lot of time and show continued engagement with his story moving forward.

So while we can’t say anything certain about Aesop-the-person, they give us something to go on for Aesop-the-figure-of-folklore. If you’re interested in that tradition, I recommend taking a look at Leslie Kurke’s Aesopic Conversations. Later chapters put Aesopica into conversation with other prose genres, but the first half is dedicated to the Life of Aesop and its interactions with different audiences, with a particular attention toward popular tradition. The introduction is specifically written for both specialist and non-specialist audiences and has quite a lot if you’re interested in methodological and theoretical issues with investigating popular culture in the ancient world.

Kurke, Leslie. Aesopic Conversations: Popular Tradition, Cultural Dialogue, and the Invention of Greek Prose. Princeton: Princeton University Press, 2011.