Understanding livestock grazing impacts on native wildlife is difficult when grazing is pervasive and has a long history. Spiti Valley in the Indian Trans-Himalaya is characterized by overstocking of rangelands and a grazing history of over 3 millennia. An intriguing aspect of the wild large-herbivore assemblage in Spiti Valley is its low diversity. In the present theoretical exploration, data on body masses of wild large-herbivore species in the Trans-Himalaya are examined against the backdrop of competition theory, to evaluate the possible role of competition in structuring the herbivore assemblage. The analysis is then expanded to include the livestock assemblage, and the possibility of the low wild herbivore diversity in Spiti Valley being a consequence of the high livestock diversity is explored. Null model analyses suggest that competitive interactions may have played a role in structuring the Trans-Himalayan wild herbivore assemblage. This is reflected in a proportional regularity in species body masses, with each species, on average, being a constant proportion larger than the nearest smaller one. Such a proportional regularity is absent in the livestock assemblage. The analyses make a case for the competitive exclusion of at least four wild grazers in Spiti over the last 3 millennia. The present grazer assemblage conforms reasonably to the theoretical predictions - these four species are among the six wild herbivores presently missing from Spiti. The implications of the analyses for conservation management in the Trans-Himalaya are discussed.