It is a very large, brownish species, with head and body length ranging from 222 to 268 mm (8.7 to 10.6 in).

In 1909, John Alden Loring collected the holotype while on the Smithsonian-Roosevelt African Expedition led by Theodore Roosevelt.

[2] In 1919, Ned Hollister provided additional information using more material, and affirmed the relationship between T. rex and T. audax.

In young animals, the crown area of the molars is small, but it grows with wear in adulthood until reaching a maximum, after which it shrinks again.

[12] In comparison to those of Tachyoryctes audax, the nasal bones are larger and have angles at the sides.

T. annectens, which is nearly as large, has smaller teeth and nasals; in T. rex, the basioccipital is broader, and the back part of the mandible (lower jaw) is better developed and has the capsule of the incisor placed further to the front.

One chamber is used for urination and defecation and to store plant matter; it produces a substantial amount of heat.