Okai, I admit I'm shipping a whole bunch of Sans in other AUs I've seen so far.
Error!Sans belongs to loverofpigges. This is her tumblr: http://loverofpiggies.tumblr.com/
Error!Sans also has an ask blog and you'll know why I drew this if you read this blog: http://askerrorsans.tumblr.com/
As for Underswap Sans, I don't know who it belongs to. If you know, please let me know so I can edit this and credit who made the AU. Thanks :3