[4] R. fergusonianus is only known from the type specimen collected in the Cardamom Hills in Travancore, a part of the southern Western Ghats in modern Kerala, southeastern India.

[3][1] The holotype of R. fergusonianus measures 32 cm (13 in) in total length (including tail), 40 times its width.

It is blackish above, and the sides are white, dotted and spotted with black.

The caudal disc is black and edged all round with yellow.

[3] The habitats and ecology of this species, R. fergusonianus, and threats to it, are unknown.