A F.A.R.M.S. annotated bibliography by John Sorenson deals with evidence for pre-Columbian animals in America. The following theories may illustrate why evidence for the "cow" and the "ox" (1 Nephi 18:25) is not perfectly clear:
1. Accelerated extinction: According to Barbara Beddall, "The horse came to Argentina in 1536 and the cow in 1556. By about 1700 according to Felix di Azara, Spanish naturalist and geographer who lived in South America from 1781 to 1801, 48 million head of feral cattle inhabited 1.7 million square kilometers. Before the middle of that century, however, wild cattle had been all but exterminated, although the human population probably did not exceed 300,000. Spaniards, Portuguese, and Indians slaughtered them for skins and fat; each Indian killed two pregnant cows a day in order to eat the flesh of the unborn calves, considered a delicacy. Throughout the year, the Spanish gauchos killed a cow for every meal (p. 4).
According to Evon Z. Vogt, "The Chiapas highlands have been populated so densely for such a long period that almost all forms of edible mammals have long since been hunted off" (p. 37).
2. Language deficiencies: According to Berthold Laufer, the Scandinavians and Lapps apply terms like ox, cow and calf to the reindeer (p. 19). [John L. Sorenson, "Animals in the Book of Mormon: An Annotated Bibliography," F.A.R.M.S., pp. 4, 37, 19]
John Sorenson comments further, "but isn't it obvious that the 'cow' of the Book of Mormon was our familiar bovine, straight out without all this hedging?" No, it is not at all obvious. First, we are trying to find out what the Book of Mormon really means by the words we have in English translation; we are not trying either to simplify or to complicate the matter, but only to be correct. In the effort to learn the truth, nothing can be assumed obvious. Second, there is a lack of reliable evidence -- historical, archaeological, zoological, or linguistic -- that Old World cows were present in the Americas in pre-Columbian times. The same is true of some of the other creatures mentioned in the Nephite record, where modern readers may feel they are already familiar with the animals on the basis of the translated names. In these cases we have to find another way to read the text in order to make sense of it. [John L. Sorenson, An Ancient American Setting for the Book of Mormon, F.A.R.M.S., p. 294]