Mixcoatlus melanurus is a pit viper species endemic to the mountains of southern Mexico.

[4][5] Adults grow to between 37.5 and 50 centimetres (14+3⁄4 and 19+5⁄8 inches) in length and have a moderately stout build.

[3] In Mexico, it is called a necazcoatl, from the Nahualt words necaztli and coatl which means "eared-serpent" referring to its "horns".

It is found in two Mexican states, southern Puebla and Oaxaca, at elevations of 1,600–2,400 m (5,200–7,900 ft).

[1] A species is listed as such when the best available evidence indicates that the geographic range, in the form of extent of occurrence, is estimated to be less than 5,000 km2 (1,900 sq mi), that estimates indicate it to be severely fragmented or known to exist at no more than 5 locations, and that a continuing decline has been observed, inferred or projected, in area, extent and/or quality of habitat.