Per Wikipedia:

Despite its vernacular name and both genera being in the same subfamily (Caprinae), the mountain goat is not a member of Capra, the genus that includes all true goats (such as the wild goat (Capra aegagrus), from which the domestic goat is derived); rather, it is more closely allied with the other bovids known as “goat-antelopes”, including the European chamois (Rupicapra), the gorals (Naemorhedus), the takins (Budorcas) and the serows (Capricornis), of Japan and eastern South Asia.