Hemimyzon is a genus of Asian hillstream loaches, small bottom-dwelling fish adapted to fast, rocky, well-oxygenated rivers and streams across East and Southeast Asia. The atlas's 14 species, ranging from about 1.5–4 in, are recorded from China, Laos, Taiwan, Vietnam, Thailand, Cambodia, and India, tracking the genus's core range through the hill streams of the eastern Himalayan foothills, southern China, and Indochina. Like other hillstream loaches, Hemimyzon species have a flattened, streamlined body and modified paired fins that help them cling to smooth rock surfaces against strong current, an adaptation shared with better-known hillstream genera in the aquarium trade.
Hemimyzon is mostly of scientific and regional interest rather than a mainstay of the aquarium trade; few of the 14 atlas species carry established common names, and detailed husbandry information for most is sparse. A couple of exceptions have picked up popular names, including Hemimyzon papilio (the butterfly hillstream loach) and Hemimyzon pengi, which has gained an online following under the nickname 'Internet Celebrity Loach.' Conservation data for the genus is thin — seven species remain Not Evaluated and four are Data Deficient — with only two species confirmed Least Concern and one assessed Vulnerable, reflecting how little formal survey work has reached many of these regional river systems.