IMPORTANT, READ THIS:
 ok, I think there's a misunderstanding about AU creations.  I think you mean about making Undertale's AU were popular.
In 2016, literally (or almost literally) everyone created an UT AU or a specific Sans AU for anything. In fact, There was a joke about creating a Sans or a AU with the first thing that was at your right side and everyone were joking and playing about it. I, when I've created Dreamtale, I made a search in tumblr if there was already an AU called like that, and I will tell a tale about this later, but I can say that in that moment there were a lot of AU's with the same name and it weren't pretty known nor popular, and even the creators didn't known each other. In fact, where I live (Spain), when you talked about Dust!Sans and Killer!Sans, a lot of people though they were a copy of each other, even if theses didn't had the same name, but if you searched about their stories, you could notice these ones didn't resembled each other.
Not because two Sans has the same or an a-like concept, it means they're equal or the same, both the creators of Dust and Killer didn't know each other when they started to make their AU's nor their Sans. As I said, there were a lot of people creating AU's in that time, that much that you could see a Sans in each corner (literally).
In fact I can also remember about two AU's sharing the same name but with a whole different stories each one and as I said before (about Dust and Killer), none of the creators knowns each other, so no one took the idea of the other one.
Having this clear, I will share the story about the name of Dreamtale: When I've created Dreamtale, and Dream Sans (because I've created Nightmare before both Dream and the AU), I've were searched on Tumblr and other social media about an AU called Dreamtale or even a Sans called Dream. I've never found any, so I started with the creation of Dream.
Almost 7 months after the creation of Dreamtale, I've found an AU that were also called Dreamtale. Curiously, I though that the creator copied the name of my AU. The funny thing where that when I checked the creation date it was created 3 months BEFORE my AU, however, when I was looking for another AU with that name, I didn't see it in another place, and I did were searching without rest for a long time. Soon I've realized there was too little posts about it, and with that too little info about that AU. In that AU, Frisk were wearing a eye cover, and as much as I could understand he could go inside in a different world (like Yume Nikki?) that were like Undertale, but in dreams, with pastel colors and everything. I've talked with that creator, and none of us were aware of the other AU existence until I talked to her.
To avoid any trouble, and to avoid that anyone told us that we were copying each other, I've discarded some ideas about Dream and Nightmare (example, the thing they could talk to you while you were dreaming).
As I said, there were a lot of Sans with equal names and with very alike designs because there was hundred and hundred of people that had a similar idea or they made an AU based on a similar thing or theme (for example, Elementale). Now, if you search about this last au (Elementale) you probably would not find too much info, or it's probably that you find a new AU with the same name, because both creators disappeared from tumblr and they left their AU's incomplete (there were a lot of people who did that) and I could say, almost without any fear of being wrong, that they never known about the existence of the other AU; maybe they now know about it, but it's very possible that, in that time, they didn't know anything.
Now, to close, I've never found another Dream, but a lot of people created Sans and drop them in short time. If this person created a Dream!Sans before me, with a similar look or something, they could have tried to contact with me when my AU becomed popular, since, in difference with any Sans that didn't had too much to search of (or almost impossible to find about it), my AU become popular pretty fast and you could easily find some info about it in Tumblr or any other media. If that dream could have been created before mine's and if it were worked alongside with a story, it could be famous, even if the designs would have been similar with mine (a good example of this is Underfell and Flowerfell).
I don't have anything else to say, more than saying this to defend myself, is to defend another people that created any AU or any Sans with similar concepts. We are A LOT of people, and it's normal that there's someone that will share similar ideas with another person, but theses will not be the same. Until now, I've never known about the creation of another Dream!Sans and I can be pretty sure that the most of the people that liked Undertale AU's didn't know, too.
Thank you for reading this, and keep being such wonderful people.
