Since that time, it has been near taboo in the Western academy to go against this kind of thinking. Before that though, equine specialists studying the taxonomy of horses favored the idea that Equus sivalensis and the probably derived Equus namadicus of India were ancestors of modern domesticated horses. A few citations:
One of these, which flourished during Pliocene times, was a slender-limbed species, standing about 15 hands high, and having a broad forehead and tapering face, and certain peculiarities of the molar teeth. This type is represented by the Siwalik horse (Equus sivalensis). The Arab may be a descendant of this stock.
-- John Arthur Thompson, The Outline of Science, 1922.
Another possible contributor to the breeds of domesticated horses is an animal
of the E. sivalensis type in the Upper Pliocene of the Siwaliks of India...
-- Henry Fairfield Osbourn, The Age of Mammals in Europe, Asia and North America, 1921.
A possible contributory to the desert breed of the Pleistocene and of
the modern domesticated horses is the animal of the E. sivalensis type of
the Upper Pliocene in the Siwaliks of India. This animal is tall, with
long, fairly slender limbs, long neck, well elevated tail, long face, which
is sirongly deflected on the cranium with a slightly convex profile and
broad brow, and teeth with a narrow protocone.
-- New York Academy of Sciences, Annals of the New York Academy of
Mr Lydekker has suggested that horses of the ' Oriental' or blood-horse type are modified descendants of E. sivalensis, while M. Boule believes the ' Occidental' varieties to have sprung from E. stenonis.
-- There is also a species of horse (Equus namadicus) which seems to be a survivor from the Siwaliks, and is allied to the existing species of the genus.
-- It used to be said that E. sivalensis could not be regarded as an ancestor of domestic horses because of the shortness of the anterior pillar of the cheek teeth. I find, however, that in some modern horses, the anterior pillars are decidedly shorter than in E. sivalensis, and that in some of the short-pillared domestic horses the face is nearly as strongly deflected on the cranium as in E. sivalensis. There is hence no longer any reason for assuming that this ancient Indian species had no share in the making of domestic breeds. But in the absence of a large and representative collection of skulls of domestic horses, it is impossible to say which modern breeds are most indebted to the large-headed, long- limbed race, which in Pliocene times frequented the area to the east of the Jhelum River, now occupied by the Siwalik Hills. Mr. Lydekker thinks E. sivalensis or some closely allied race " may have been the ancestral stock from which Barbs, Arabs and Thoroughbreds are derived."
-- Some paleontologists believe that the Indian species (E. sivalensis and E. nomadicus) became extinct, and that E. stenosis through E. robustus, gave rise to the modern breeds. But the presence in the Java, Sulu, and Borneo horses of the above-mentioned vestiges of the preorbital depression and the large premolars make it appear that some modified descendants of E. sivalensis survived, and that that species was the lineal descendant of the Hipparion.
Mr Lydekker has recently discovered in the skull of the modern Indian horse a vestige of the pre-orbital depression or pit which must, as in living deer, at one time have sheltered a tear-gland. He regards the Arab and the English thoroughbred as distinct from the Equus caballus of Western Europe, of which the original tint seems to have been dun with black marking on the legs, and sparse development of hair on mane and tail, like the Kathiawar breed. The Arab variety, with a bright bay colour, white star on forehead, and not in frequently a black bar round the fetlock, is quite different. It is almost certain that the Indian horse is a descendant ofthe fossil horse of the Siwaliks, in which the skull pit is larger. A similar pit was found in the skulls of "Eclipse," " Bend Or" and " Stockwell," and hence it has been concluded that the Arab stock from which our thoroughbreds are derived was originally imported in ancient times from India.
-- William Crooke, Things Indian: Being Discursive Notes on Various Subjects Connected with India, 1906, p. 253.
E. sivalensis is also known for its large head in proportion to its body, and for a convex facial profile.
The convex facial profile is strongly evident in the front horse in the Sunga era sculpture below from the Huntington Archive , and in the horse of the following image from Konarak.