Final answer:
The 'El Castillo' pyramid at Chichen Itza is famous for the appearance of a feathered serpent shadow during the equinoxes, symbolizing the descent of the god Kukulkan.
Step-by-step explanation:
The phenomenon associated with "El Castillo" at Chichen Itza during the equinox is the appearance of a feathered serpent. This occurs at the spring and fall equinoxes when the setting sun casts a shadow on the stairways of the pyramid, creating the illusion of a serpent slithering down the steps. This serpent aligns with the carved serpent heads at the base of the balustrades, symbolizing the descent of the Maya god Kukulkan, also known as Quetzalcoatl.