To expand on the unique adaptation to the tropical environment, something likely developed while the species was still wild over tens of thousands of years, I will list some old references to the horse in the Philippines.
The Philippine Horse is primarily a development of the Sulu Horse. In central and northern Philippines, the horse has some admixture with Spanish mustangs and the Chinese horse, but still its characteristics mostly resemble the Sulu Horse of the southern Philippines. The idea that the horse in the result of the deterioration of Spanish breeds like the Barb has been dealt with in various works.
However, unlike in the South where indigenous names like kuda still survive for the horse, the indigenous names have been replaced in the North. This is not a phenomenon limited to the horse though. The bolo, for example, is a Philippine blade with a name derived from Spanish. Philippine stick-fighting also in many areas goes under names "arnis" and "escrima" both derived from Spanishi words although in the South the indigenous or regional name "kali" is still used. Terminology related to horse-riding and horse care does survive however, and it is possible that an indigenous horse name has survived.
The words tigbalang, tikbalan and similar cognates have as their primary meaning through much of the northern and central Philippines, the name of a mythical creature that is usually described as either part-horse, part-human or as a supernatural horse. Possibly this previously was also a word for "horse" before displacement with the Spanish word.
Here is a list of notices that highlight the high adaptation of the Philippine horse to the tropical environment:
The horse of the Philippines is a descendant of the Sulu horse and the horses brought by the Spaniards from Mexico and China. Although it is a small animal, probably no other breed of horses in the world has the combined qualities of style, action, vigor, and endurance to the same degree that the Philippine breed has. This has doubtless come about from the little attention given to these animals ; and thus, by a process of natural selection, those have survived which are best fitted to endure the conditions of Philippine life. The Philippine horse is used for riding and light hauling. No heavy wosk in the field or on the road is performed by it ; cattle and carabaos are used instead. In mountainous regions horses are often utilized as pack animals.
-- Hugo Herman Miller,
The Philippine horse is small and under-sized but it is well-developed, and not only is it physically well- proportioned, but its great ability to withstand heat as well as its enormous endurance are widely known. Only recently, a gift of the six best available specimens of the Philippine Horse was graciously accepted by the Imperial Household.
-- Manila Sinbun-sya, The Official Journal of the Japanese Military Administration, 1942.
The Philippine pony is used for all light draught for which the caribou or the mule would be inappropriate. These little animals are not much larger than the famous Shetland ponies, but they seem to be more like the mustang or Indian pony in their habits and general make-up. Tough as leather, wiry, and sure-footed, they have wonderful endurance, and they thrive in this climate where the larger horses of Europe and America can be kept alive only with great difficulty.
-- Frank Wiborg, The Travels of an Unofficial Attaché: Descsribed in Simmple Narrative, 1904.
We have a wonderful horse in the Philippines, of almost unequaled courage and spirit and good confirmation, but he is too small for general use.
It is said that no other horse in the world has combined the quality of style, action, vigor and endurance in one "wonderful piece of horse flesh."
-- Pacific Science Association, Proceedings - Pacific Science Congress, 1967.
It is often said, and by people who know, that the Philippine horse or pony is the best piece of horse-flesh in the world for his inches.
-- David C. Kretzer, How to Build Up and Improve a Herd Or Flock, 1930.
In many respects, these ponies are the best specimens of horse flesh in the world, being possessed of wonderful endurance and a remarkable combination of quality, action and vigor.
-- Philippine Islands Bureau of Agriculture, The Philippine Agricultural Review, 1915.
The horses of Insular Southeast Asia are often found in the wild as feral horses where they endure conditions of weather and exposure to disease and parasites that appear surely the result of long periods of tropical adaptation.