In the Lao tradition, there is a story that when the Buddha was still alive and establishing the monastic tradition, a nak wished to be ordained as a monk.
The nak transformed himself into a human being and the Buddha let him keep this form to enter the monkhood. But one day, the nak fell asleep, and during his nap the spell wore off. The Buddha was obliged to ask the nak to leave the monkhood.
Saddened, the nak agreed, but asked the Buddha that in the future, someone preparing take the vows to be ordained as a monk would be called a nak, in remembrance of the nak faith in the Buddhist teachings. The Buddha agreed, and the rest is history.