Mongseng
Mongseng is a plane that is depicted by the card Kharasha Foothills from Planechase (2012). The card shows warriors on a battlefield, with flags on their backs bearing resemblance to the sashimono-carrying samurai, and more generally being reminiscent of traditional Chinese warrior culture. The depiction of large steppes and the name Mongseng might be references to the Mongols.
Connexion with Tarkir
Mark Rosewater has stated that the planes showcased in Planechase (2009) and Planechase (2012) were created as potential locations for future stories.[1] When Khans of Tarkir was revealed at San Diego Comic-Con 2014, Kharasha Foothills was mentioned as the source of inspiration for the plane of Tarkir.
While the two planes are technically distinct, Mongseng was basically rebranded to Tarkir, just as Arkhos has been rebranded to Theros.[2] It is therefore unlikely that Mongseng will be explored further, as its basic concept and flavore are now used for Tarkir.
Mark Rosewater has stated that bigger factors than continuity led to the name change, which prompted a commenter to observe[3] that the earlier name could be read as containing a slur. More importantly, the process for choosing a card name is easier that choosing the name of a plane, as the name of a block has to be able to be deposited as a brand.