from the website of the British Museum:
Head of a horse of Selene from the east pediment of the Parthenon
One of the best loved sculptures from the ParthenonThe east pediment of the Parthenon shows the birth of the goddess Athena from the head of her father Zeus. The event was witnessed by various figures shown on either side and filling the triangular space of the gable end of the temple. In the very corners of this triangle, the time of day was set by the chariot of Helios, god of the sun, rising at dawn, and the chariot of Selene, the Moon goddess, sinking beneath the horizon. Selene's torso is in Athens, while the head of one of her team of horses is in the British Museum.
This is perhaps the most famous and best loved of all the sculptures of the Parthenon. It captures the very essence of the stress felt by a beast that has spent the night drawing the chariot of the Moon across the sky. As the unseen vehicle was shown sinking low in the west, the horse pins back its ears, the jaw gapes, the nostrils flare, the eyes bulge, veins stand out and the flesh seems spare and taut over the flat plate of the cheek bone.