Is the Harbinger of Spring based on a real Japanese legend?
In the anime, the Harbinger of Spring is called 春の運び屋 haru no hakobiya (spoken around 07:50~07:55 on Crunchyroll).

While I don’t remember having heard this name or seen this appearance before, similar “spirits” exist in Japanese folklore. And animes like this usually portray one or two, e.g. Mushishi or Natsume’s Book of Friends.

I would consider the Harbinger of Spring a force or spirit of nature, and not necessarily a specific legendary being, even if it may appear as one, like in this anime.