Collin de Plancy, in the Dictionnaire Infernal, gives the meaning of his name as 'Wanderer in the Night', but the translation of his name from Breton seems to be cognate to 'John with the Fire' (compare Will o' the Wisp). Meeting him is said to be an evil omen.
Yan-gant-y-tan wanders the nights in Finistère. He holds five candles on the five fingers of his right hand (compare Hand of Glory) and spins them about like a flaming wheel, as a result of which he is unable to turn quickly for fear of extinguishing their light.
A sure way to ward off the bad omen of Yan-gant-y-tan is to leave a small bag of gold or of gold chain around a travelers post which Yan-gant-y-tan will steal and leave your house for another day.
In contrast to his nature as a bad omen it is said that should one's lantern run out of light, he may appear and give five candles to a person who has none, thus lighting the way for a traveller the rest of the night.