It's kind of odd to think about how for Euclid, Pythagoras was kind of a folk story. A guy with a possibly math themed mystery cult. He lived 270 years before Euclid. Kind of like a George Washington? With stories about chopping down cherry trees that might not be true.
The theorem that's named for Pythagoras is even older. A *lot* older. Even to Pythagoras and Euclid Egypt was "ancient" and that's why there is a story that Pythagoras went there to study 'the ancient mysteries.'