Article: While the Ming dynasty traded horses with Tibet, it upheld a policy of outlawing border markets in the north, which Laird sees as an effort to punish the Mongols for their raids and to "drive them from the frontiers of China." However, after Altan Khan (1507–1582)—leader of the Tümed Mongols who overthrew the Oirat Mongol confederation's hegemony over the steppes—made peace with the Ming dynasty in 1571, he persuaded the Ming to reopen their border markets in 1573. This provided the Chinese with a new supply of horses that the Mongols had in excess; it was also a relief to the Ming, since they were unable to stop the Mongols from periodic raiding. Laird says that despite the fact that later Mongols believed Altan forced the Ming to view him as an equal, Chinese historians argue that he was simply a loyal Chinese citizen. By 1578, Altan Khan formed a formidable Mongol-Tibetan alliance with the Gelug that the Ming viewed from afar without intervention.

Question: Who did the Ming trade horses with?
Tibet