POD: Ancient Egypt gets its own version of
Leslie Fish, and the Cult of Bast begins breeding temple cats for intelligence and polydactyly.
Extra points if this is partially triggered by the appearance of a furrless spontaneous mutation (like Sphynx cats) that makes them seem closer to human, or if the later appearance of such a mutation is seen as divine favor for the work.
Furlessness optional, within a few generations (human generations- many more cat generations) you end up with a large breed of highly trainable six-toed cats with opposable thumbs, and similar intelligence to a capuchin or snow monkey. Perhaps some of them are used as special guards for the Pharaoh, seeing as how Bast was supposed to be a defender of Ra.
Three questions:
First, are there any particular times at which this would be more likely to occur than others?
Second, what other traits would Egyptians be likely to breed for?
Third (and this is rather open-ended), what might some other effects on Egyptian society, or the world at large if/when they hear about it? Perhaps some particularly bright priest works out the rules of Mendelian genetics a few thousand years in advance from studying birth records?