I've been curious about that bit of "concept art" for a few years myself, so your post prompted me to go on a wild goose chase.
Having spent most of the last three hours brute forcing ダークライ + relevant kanji phrases (設定, 足, 尻) through some search engines, narrowing down results to between 2007 and 2015, and then looking through the results, I am 90% confident in saying that there is no image of the butt edit available online which predates the post made by fishof-blog that you reblogged from May 2014. If it is anywhere at all, it almost definitely wasn't posted in English, there is a slim chance it was posted in Japanese, and it's probably only available on an archived form of whatever website it was originally posted to. I can say without a shadow of a doubt that it didn't originate on Twitter, as there are no instances of the image on there that I could find predating May 2014.
The biggest bit of news I have: the image *definitely* isn't real and *definitely* comes from an edit a different page than the one you posted earlier of the film's settai, 401_04. There was an auction in 2023 of some production materials from the film on Mandarake which includes that page: https://k.mandarake.co.jp/auction/item/itemInfoJa.html?index=702045
My two hunches are that it was either made by fishof-blog and that their posting of it in May 2014 (reblogged by cherrycovetc; apparently deleted later by fishof-blog, who then reblogged smearg-archive's post of the same image) was the genesis of the butt edit, which was popularised when reposted by smearg-archive -- every English mention of darkrai's alleged ass that I could find dates from after June 9th 2014, the date of smearg-archive's posting of it -- or that it was posted to a forum which was most likely Japanese and then cross-posted for the first time on the English internet by fishof-blog. I lean towards the latter, because fishof-blog seems to have been an authentic blog, albeit one that was briefly used, but I don't think there's any way of definitively answering that one. Unless one were to somehow get in touch with, and receive a definitive answer from, a blog which seems to have only been active for a couple of weeks 12 years ago, or had a much better grasp of Japanese and of searching through archived web pages, as well as more time to dedicate to it, than I do.
I think you are correct in suggesting that smearg-archive's posting of the image in June 2014 is what led to it eventually becoming such a prominent myth -- I even saw it pop up on an Instagram reel by a generic pokémon account which had about 30k likes while doing this research. I wonder if whoever first made the edit sees how big it's gotten, from time to time, and has a good laugh to themself about it; it's a very well-done hoax!
Ah, so you weren't the only one playing around with Japanese search terms! When I couldn't find anything under it's normal Japanese name, I turned to using ディアルガVSパルキアVSダークライ, which is part of the title of the Japanese movie. I also got some listings for the movie materials, and like you, did not find any that had Darkrai's totally real ass.
It popping up on a generic Pokemon account and being passed as real does not surprise me. It's rampant on Twitter, and if you browse the Darkrai tag here on tumblr, you will find several users who believe the edit to be real as well. Which, I almost can't blame them, because tracking these settei and concept sketches of Darkrai can be a pain in the ass since they're not something that's going to magically pop up on Google search results...well unless it's the ass edit, of course.
I will admit though, I did some surface level sleuthing at first and came across this Korean blog that has a picture zoomed in on it. The caption is very funny:
Unless someone else comes in with an even earlier source of this image, I can agree with you that this is a very well done hoax. This edit has gotten around so much, I have seen a piece of fanart someone did of Darkrai standing on something and there's a breeze that reveals the Hostess Factory, or you have the people that crush on Darkrai and/or want to fuck it and froth over that image like their life depends on it (I saw an instance of this on r/Losercity during one of my google image searches). All it took was one tiny edit from some guy in 2014.
Honestly, I'm glad someone else decided to dig into it alongside me, because you don't find many people who try to tell others "hey that isn't real here's all the material available there is no ass". Not even the most popular Pokemon information/archival accounts will do their due diligence and correct people about it. Kinda annoying, but what can you do.
TL;DR: 10/10 hoax, can't believe some guy sat down and decided to give Darkrai an ass, we truly live in a society
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