500 Arhats
According to scriptures, there was a group of 500 arhats who assembled to recite the general teachings of the Buddha, at the First Council, which is said to have taken place shortly after the Buddha went into nirvana.
The bodhisattvas assemble in order to preserve the Mahayana teachings. There was a requirement that only arhats could attend the First Council, to preserve the authenticity of the teachings.
One of the Buddha's disciples, Ananda, had not attained nirvana at that point (so was not an arhat). He was about to be excluded from the Council. He spent the night in meditation and just as he had given up, and was laying down to rest, he attained nirvana at the moment his head touched the pillow.
This story is very important because Ananda was the only person who had heard all the Buddha's teachings and also, he remembered them so perfectly that all the sutras which begin "Thus I have heard" were recited by Ananda. This story also shows that even though Ananda had spent his entire life serving the Buddha as his personal attendant and so had not had time or opportunity to practise meditation, when he turned his mind to it, he attained realisations so easily.
The 500 Arhats known and revered in East Asia seem to refer to these same 500 disciples. Their names have not been preserved, except for the closest disciples, who were historical persons, and the number 'five hundred' indicates a great number rather than actual headcount.
The 16 Arhats we are more familiar with were actually part of that first group of 500 (or great number) who attended the First Council, but they vowed to renounce nirvana in order to devote themselves more effectively to the relief of human misery, like bodhisattvas.
These relics were offered by Arya Hwang-Thaweevitd who collected from temples around Bangkok and offered these during a relic display in La Puente, California in 2004.

