Fossil remains of the Gigantopithecus have been found in India and China, and they date back from 3,000 years to 12 million years ago. During this period, the Himalayas were rising by as much as 2500/3000 meters. Because of this increase in height, many species of animals, including the Yeti's ancestor may have become isolated.
Based on the fossile remains, scientists are able to give a good estimate on how the giant ape would have looked. They believe that the ape would have stood about 3 meters (About 9.8 ft) tall, and with an arm span of over 3.6 meters (12 ft)!
Some experts do argue that the famous Yeti's footprints have all been found above the snowline, meaning that it could not have been the Yeti's ancestor. Above the snowline lies a bare terrain that is unable to support the life of large mammals, including Yetis. They say for the Yeti to have existed, it must have lived down in the lower forested valleys of the Himalayas. Here the vegetation is far more dense and regularly gets foggy. This means that there were less human inhabitants to disturb them.
So although there are theories to where to Yeti came from, or what it might have been, I guess we can never truly know for sure.