Barrier islands represent unusual and fragile ecosystems, influenced by multiple physical stressors and populated by organisms that are resource and habitat specialists. Dung beetles (Coleoptera) provide essential ecosystem services through the removal of dung from the environment. Their role in the unique environment of a barrier island dominated by a large non-native grazing species has not been widely investigated. Using linear transects of pitfall traps baited with human dung, we examined the species diversity of dung beetles found in three habitats (marsh, dunes, and forest) of Assateague Island, a barrier island off the coast of Maryland and Virginia, USA. Fifteen species (13,626 individuals) were collected. Estimates of species richness suggest there may be an additional 3–6 species on the island that were not sampled in this study. Heterotrophic succession of dung beetles attracted to horse dung was also examined and compared between Assateague Island and mainland horse farms. Observations were made at 1, 3, 5, 7, 14, and 21 days without replacement of bait. Significantly fewer dung beetles were attracted to Assateague Island horse dung placed on the island compared to mainland horse dung placed on two horse farms, 80 and 748 beetles, respectively. No beetles were collected after seven days of the study. When the horse bait was switched with island-origin horse dung placed on two mainland horse farms and dung from horse farms placed on the island, there was no significant difference in the number of individuals collected. To our knowledge, this is the first research that has examined the attraction of dung beetles to feral horse dung.