Today February 2 is known as Twin Tail Day. No particular reason why as to the origin of this day on this date but the Japan Anniversary Association‘s explanation is because of how 2/2 overlap with each other donating the Twin Tail.
But I think the real question here is, how did the word “Twin Tail” came about? Sure, we all know it refer to the hairstyle in which both side of one’s hair is tied into 2 ponytails. Officially this kind of hairstyle is call Pigtail in English. The word Twin Tail is just another word made up by the Japanese.
The earliest origin of this word can perhaps be traced back to the “The Return of Ultraman” in 1971. In one of the episodes, Ultraman battles a monster call the Twin Tail.
That’s all I know. If there’s other theories, sound out in the comments.