Zenos is a prophet from the plates of brass, but not one who is recorded in the version of the Old Testament that has come to us through history. The brass plates were kept by descendants of Joseph of Egypt, and they had their inheritance in the northern kingdom of Israel. The Old Testament as we have received it came through the southern kingdom of Judah. That division had occurred over three hundred years before the Book of Mormon story begins. Thus, there were three hundred years in which the northern kingdom had their own prophets, recorded in their own records, but who were distinct from both the prophets and records kept in the kingdom of Judah.
While we are accustomed to a specific collection of books that we call the Bible, before there was such a collection there were multiple copies of separate books which were later combined into a canon. There were other writings that were left out. The most commonly known set of writings are known as the Apocrypha, a set of pre-Christian books accepted by the Catholic Church, but not accepted by Protestants and those influenced by Protestant ideas. The fact that there was no specific authorized canon meant that the northern kingdom scriptures kept among the descendants of Joseph could be brought to Jerusalem and then added to with the southern prophets after its arrival. It is in that way that we have the lost northern kingdom prophets Zenos and Zenock, and still have the southern kingdom prophets such as Isaiah and Jeremiah.