The specific epithet rhombicaulis is formed from the Latin words rhombicus, meaning "rhomboid", and caulis, "stem".

Nepenthes rhombicaulis was first collected by Shigeo Kurata on March 29, 1972, on Mount Pangulubao at an altitude of between 1700 and 1900 m above sea level.

[2] One of the original specimens, Kurata 4300, was designated as the holotype of the species and is deposited at the herbarium of the Nippon Dental College (NDC).

[6] Kurata's illustration of the type specimen shows a small apical appendage on the underside of the pitcher lid.

However, Matthew Jebb and Martin Cheek pointed out that this feature is not present in the isotype held in Singapore.

Observations made at the type locality by Charles Clarke and Ch'ien Lee seem to confirm this; while some wild plants exhibit this appendage, most do not.

[6] In the 1983 book Carnivorous Plants of the World in Colour by Katsuhiko and Masahiro Kondo,[3] a photograph of N. gymnamphora is identified as N.

[8] In 1993, Bruce Salmon postulated that the lower pitchers of N. rhombicaulis are specially adapted to trapping subterranean insects.

[9] Rudolf Schmid-Höllinger reported observing upper pitchers of N. rhombicaulis on Mount Pangulubao in 1993.

[6] Based on Schmid-Höllinger's observations, they are ventricose in the lower parts and elongated above, becoming tubiform or slightly infundibuliform towards the mouth.

Lower pitcher range in colour from dull green throughout to light red with purple blotches.

[10] Nepenthes rhombicaulis is known from a number of peaks in the Indonesian province of North Sumatra, particularly around Lake Toba.

Plants that appear to match the description of N. rhombicaulis have also been recorded from Mount Bandahara in Aceh.

[6] Lower pitchers frequently develop embedded in detritus and leaf litter on the forest floor, resulting in their often deformed appearance.

He postulated that the species is adapted to trapping subterranean insects, although he did not examine the contents of these pitchers to test the hypothesis.

[6][11] In 2001, Charles Clarke performed a cladistic analysis of the Nepenthes species of Sumatra and Peninsular Malaysia using 70 morphological characteristics of each taxon.