What creature was half-horse and half-man?

The correct answer is the Centaur. In Greek mythology, Centaurs are depicted as creatures with the upper body of a human and the lower body of a horse, symbolizing a dual nature that combines the intellect and culture of humanity with the wildness and instinct of a horse. This hybrid form is significant in various myths, where Centaurs often embody themes of balance between civilization and primal instinct, and can represent both nobility and chaos depending on the story.

The other creatures mentioned have distinctly different characteristics that set them apart. The Hydra is a multi-headed serpent known for its regenerative abilities; the Gorgon, such as Medusa, is a female creature with snakes for hair whose gaze could turn people to stone; and Satyrs are companions of Dionysus, often depicted with human upper bodies and the legs, ears, and sometimes tails of a goat, featuring a more playful and mischievous nature. Each of these creatures has unique attributes that do not relate to being half-horse and half-man, confirming why the Centaur is the only correct choice for this description.