Technically,
Pythagoras did not think up the
theorem. Nor did he
prove it. Nor did he
exist in the first place. But:
had he existed, and
had he known about the theorem, he would probably have been last person on earth to realise it. Thus it is
logical that it is named after him.
In fact the Pythagorean theorem was known by many people before the
Greeks, eg the
Chinese and the
Babylonians.
There is no evidence that it was known to the
Egyptians though, a popular
myth that has also been
noded here. It appears, however, that they did know that triangles have three sides, and in the early 20th century a
Belgian historian concluded that they might have known the Pythogorean theorem. After a few references that nobody bothered to check this became an accepted
truth.