Uwan - うわん - [Yokai]
• About this yokai: Best described as a "disembodied voice" - This horrible yokai is often depicted as a wrinkly creature with blackened teeth & sharp claws, it is posed with arms raised in a scary way as though its jumping out at you!! - it is often said that it Inhabits ruined houses, or that it lives on abandoned temple grounds. it when you least expect it, it screams "UWAN!!!" from somewhere in the house, at the top of its lungs to scare away any trespassers, but no matter where you look, you'll never find the source of the yell.. According to some, they live in graveyards, and yelling back at the creature causes them to flee, but if you don't yell back at the creature immediately, it will shove you into a coffin and steal your soul!!!
• History, Early Illustrations & Similar yokai:
The earliest picture scrolls featuring this yokai are plentiful, from the edo period (1603-1867) titled Bakemono no e (化物之繪, "Illustrations of Supernatural Creatures") to name a few well-known scrolls: "Hyakkai Zukan" , "Bakemono-e", Bakemono Zukushi" & of course toriyama sekiens, gazu hyakki yagyo . - like most of the other yokai featured in these bestiary scrolls, Uwan is presented without any context aside from the name & a nearly identical pose. - [It's often thought that these yokai would have had stories that where obvious to know at the time, but perhaps they fell out of public interest over time and were seemingly, lost to history.] - Only the top half of the creature is ever depicted, many variants exist with the same monster but in many different colors... However, Without any written context, it's very difficult to say what kind of yokai it is actually supposed to be! No earlier written record of this yokai's physical appearance exists and It wasn’tuntil these scrolls came about that the yokai was given that Uwan were considered to be anything more than phantom sounds.
In general, Uwan is very oni-like with just 3 clawed fingers to each hand (often said to represent Buddhist vices.) a snarling mouth with Ohaguro. (Black teeth, Teeth blackening is a regular thing sehared between these early yokai too.) he appears to be wearing a cloth below but because the bottom half is not depicted its hard to say for sure.
it's often pointed out that in certain parts of kumamoto & Kagoshima prefectures "wan" or "Wanwan" was a regional dialect & a generic term which meant "monster." - However, even with that, its unclear if this dialect actually had anything to do with why the monster in question was why it was named this. (but i'd say its a pretty good guess.) As similarly to Uwan, a hag-like yokai called "Wauwau"(わうわう) - (better known & later interpreted as "Ouni.") exists in the same early picture scrolls as well. Notably, "Wau-wau!" could also be read as an monsterous yell.
[Uwan - Hyakki Zukan 1737]
Although most simply say that "it inhabits abandoned buildings" The idea that the Uwan "haunts old houses" seems to originates from Toriyama sekiens illustration in his Gazu hyakki yagyo  which features uwan popping out of willow tree over a a dilapidated building. - Note that again theres no explainatory text included with the entry.
..As yokai expert Shigeru Mizuki mentions in his yokai encyclopedias, reliable information about "Uwan" is very scarce, and very few early materials can actually be found on the subject, most legends about the creature where seemingly invented in the modern era (70s-80s) for instance: An "Uwan" is seemingly Included in '東北怪談の旅' - "A Journey through Tohoku Ghost Stories" [published 1974] in it there is a story from Aomori prefecture where in an unseen monster yells "Uwan!" at a old married couple all night long. - This book is indeed a collection of ghost stories from the Tohoku region, but many of the stories included in that book have been found to have simply been made up by the author. - According to Mizuki, This story appears to be no exception. [it also very well could have been spread around orally before the books publishing.] -- I digress, without other earlier written accounts to be found, its certainly hard to say! - Mizuki wound up using that story for his bestiary entries about the creature regardless, so it stuck.
As well the idea that this yokai 'lives in graveyards' seems to have originated as a rumour originating from Arifumi Sato in their [1980s] yokai encyclopedia's. - Sato didn't give a proper source for his story, so again, its unclear what they might have based their uwan story on. - Given Satos record for inaccuracies and seemingly making stuff up, we can assume it was likely just to add something to the creatures lore. - Plus; "if you yell back at it, you'll be ok, but if you don't you'll..." has a distinct urban legend flavor which reminds me greatly of kuchesakeonna & other modern japanese urban legends, popular from the 70s 'til today.
--But i digress, reguardless of the "truth" - these are fun stories to add to such a vague creature, and "the broken telephone of time" is a matter of fact, the epitomy of how folklore works!!! - I do like to think of uwan as an audio phenomena for unkown noises, sudden yells and house creaks, similar to Yanari or Azuki Arai. There are other similar monsters that throw their voices/make noises to fool/scare humans or exist as an explaination for odd/scary nature sounds.... At the end of the day, who can say for sure? it's yet another a yokai surrounded in mystery!!
• Similar critters: "GOWROW", Treesqueak.