I’ve seen mention of a rather mysterious language spoken in western Mongolia in some of the literature: Khoton. As far as I can tell, it is attested in only one document: Владимiрцовъ, Б. and А. Самойловичъ. 1916. “Турецкій народецъ хотоны.” Записки Восточнаго Отдѣленія Императорскаго Русскаго Археологическаго Общества 23, 265-290.
I was finally able to find a copy of this article online (thanks in no part to the shoddy citations many authors have given), and have started to add forms to the database. To say the least: it’s weird. A lot of the literature has used Khoton as a justification for inserting vowels at the end of certain reconstructed forms, because some of the forms given in the aforementioned publication have unexpected short vowels at the end of certain words. There’s some justification for this, as certain Mongolian correspondences with Turkic have unexpected final vowels as well. However, I’m skeptical that these vowels in Khoton are actually significant, as it would be very strange that only one poorly known variety of Turkic would exhibit these vowels, while others have lost them. These same short vowels appear in the forms for “6” and “7”, yet were not lost in any other Turkic language (even though the resulting forms would still be pronounceable).
As I work through this pre-reform Russian document, I hope to be able to figure out where Khoton fits in the overall family tree. It may be related to Chanto Uyghur, another poorly attested language of Mongolia that is currently spoken in the same aimag. I’ve suspected for some time that Karluk might be a wastebin taxon given that many of the non-standard varieties of Uzbek and Uyghur meet none of the criteria for being in that family, and the Khoton data might be a further nail in that coffin.