Photo of an Ichthyocentaur from frescoes at Villa del Casale in Sicily sent by a twitter follower! The artworks there date from c. 4th-5th C, and although the site is mostly known for its mosaics, the frescoes from both interior and exterior walls are pretty extraordinary. The artists who created the mosaics and frescoes in this Roman Villa were African, and their work was preserved courtesy of a mudslide that buried the building around the 12th century.
Seriously though, if ever you need proof that humans can come up with some pretty crazy sh!t all on their own, look up any medieval (or older) bestiary ever.
Honestly it’s one of my favorite pastimes! You can check out a guided tour of one of the British Library’s medieval bestiaries here that explains what they are and gives a lot of good info: http://www.bl.uk/turning-the-pages/?id=32841b35-03d4-40d5-a218-0e9effb1843a&type=book