Apparently, according to my Archaeological Study Bible, the ‘wild ox’ in Job 39:9 is probably an extinct type of wild cattle called an aurochs. They were much larger than most domesticated cows. According to Wikipedia, they went extinct in 1627 AD.
Wikipedia also says:
The wild-ox called re’em (Strong’s # 07214) in the Bible (Numbers 23:22 and 24:8, Deuteronomy 33:17, Job 39:9-10, Psalms 22:21, 29:6, 92:10 and Isaiah 34:7) is occasionally associated with the aurochs and has incorrectly been translated as “unicorn” in the past (The International Standard Bible Encyclopedia, Entry for ‘Wild Ox’, Copyright, 1939, by Wm. B. Eerdmans Publishing Co.).
Richard Venable