nurarihyon
A type of traditional youkai. It is also often interpreted as "The Supreme Commander of Youkai". In old literature, this name was only one of the common names for the mysterious slimy monsters. In some old folklore stories, a slippery sphere that appears in the sea was called Nurarihyon.
For unknown reasons, in the 18th century, this creature came to be portrayed as an old man with a large bald head like a gourd, wearing a kimono or monk's stole. Toriyama Sekien drew a picture of this youkai in his 1776 book Gazu Hyakki Yagyou, and the folklorist Fujisawa Morihiko introduced it in his book published in 1929-30 by adding unsourced commentary to it: まだ宵の口の燈影にぬらりひよんと訪問する怪物の親玉 ("The boss of monsters unexpectedly slips into someone's house, being illuminated with a lamplight in the early evening.") A variety of people such as manga artists, novelists, and occult writers expanded the interpretations of this short paragraph by adding almost random commentary. Mizuki Shigeru integrated these commentaries and circulated it in his manga and anime, and thus the current Nurarihyon was born.
Nurarihyon in present times is explained in the following manner: He enters one's house from nowhere during the evening, and behaves like it's his own home drinking tea and tobacco. Because people do not notice him since they are too busy, or mistakenly think that he is the landlord, they cannot expel it.