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Wild horse management on western rangelands is an ecological and sociological issue of escalating concern. Identifying potential dietary overlap between horses and livestock and wildlife will help inform management decisions to optimize multiple interests and guide innovative research questions. We conducted a review of microhistological fecal diet studies for wild horse, beef cattle, domestic sheep, elk, pronghorn and mule deer diet composition on interior rangelands of western North America. Our search yielded 33 studies from 12 states and 1 province with 197 unique species-season samples. To understand broad ecological interactions, we summarized plant species into graminoid, forb, and shrub functional groups. Using Principal Components Analysis (PCA), we compared functional group diet composition of these six herbivores for four seasons (spring, summer, fall and winter). The first PCA axis was always explained by the grass to shrub gradient between grazers and browsers except in the summer when forbs were equally as influential as shrubs. The first axis explained 58% to 72% of the variation depending on the season and the first two PCA axes explained 90% to 92% of total variation. Unexplained variation in the ordination was attributed to inter- and intra-annual precipitation variation and variation across study locations in precipitation and relative availability of plant functional groups. In the spring, wild horse diets only overlapped with cattle and in the summer wild horses only overlapped with cattle and domestic sheep diets. Fall and winter wild horse diets overlapped with cattle, sheep and elk with potential overlap with pronghorn. Wild horse diets never overlapped that of mule deer regardless of season. Wild horses consistently selected for graminoids and displayed the greatest functional group diet niche breadth in winter. Season, plant composition, and herbivore assemblage may all influence dietary competition between wild horses and other large herbivores sharing interior North American rangelands.