Cook's Robber Frog is nicknamed "Demon of Puerto Rico" because the native people are afraid of its call and its phantom-like appearance. The native name guajón is derived from its habitat, the so-called guajonales. These are caves formed by rock formations between huge stones.
The species was first described by Chapman Grant in 1932. Females are slightly larger than males. Females can reach a length of 5.11 cm and males can reach 4.3 cm. The color is solid brown with whitish underparts. They have a yellow throat and abdomen. Therefore it is the only Eleutherodactylus species which exhibits sexual dimorphism in color and size. Both sexes have large, white-rimmed eyes and large, truncate disks under their feet. It is endemic to the grottoes and the rivers of the Pandura mountains range in southwestern Puerto Rico.