King Rhesus of Thrace was a Trojan ally in Homer's Iliad. His name has been given to a type of monkey from the macaque family, native to Afghanistan, Pakistan, India, China, Myanmar (alias Burma), and a few neighboring countries.
The trivia items below may give away important plot points.
During shooting of the scene where Hercules breaks free of his chains, Dwayne Johnson stated that he "blacked out" after every take: "I asked the prop department to lock me in with real steel chains that I couldn't break, so the performance would be real. When Hercules finally accepts his fate of being the son of Zeus, it's the epic moment of the mythology - and our movie. We shot this scene 8 times - I blacked out every time. Down to my knees. Well worth the pain.. this moment is iconic. I'm excited for you to see it."
When King Eurystheus congratulates Hercules for killing the Hydra, he's shown breeding peacocks. This is the sacred animal of the goddess Hera, Hercules' arch nemesis. Possibly a clue to his true intentions.