This list of mythical creatures that begin with this letter and I am constantly gathering information from each continent, and is continually being updated.
Nurarihyon (Japanese)
They are considered a monster that cannot be caught. One can find that it often appears in the Edo Period, but any further details about it are unknown. In folktale legends, they are a member of the Hyakki Yagyo, and there is a type of umibozu in the Okayama prefecture that can be found under that name, but it is not clear whether they came before or after the "nurarihyon" in the pictures.
Yhey are round about as big as a human head that would float in the Seto Inland Sea, and when someone tries to catch it, it would sink and float back up over and over to taunt people. Presently, it is thought that these are portuguese man o' war, spotted jelly, and other large squid and octopuses.