Ramakian is the Thai version of the Ramayana mythological epos. The most famous version is the one written by King Rama I from 1807, a shorter version was written by King Rama II.
While the main story is identical with the Ramayana, many other aspects were transposed into a thai context - for example the clothes, weapons, nature are described as being thai style. Even the names of the characters were changed, like the god Narayana which is called Narai.
The murals in the Wat Phra Kaew depict the whole story of Ramakian, as well as many of the statues depict characters from it.