Four ancient prophets whose writings do not appear in our present Bible are mentioned in the Book of Mormon. These four prophets and the Book of Mormon references to them are as follows:
(1) Zenos, who is mentioned 12 times in the Book of Mormon in 1 Nephi 19:10, 12, 16; Jacob 5:1; 6:1; Alma 33:3, 13, 15; 35:7; Helaman 8:19; 15:11; and 3 Nephi 10:16.
(2) Zenock, who is mentioned five times in the Book of Mormon in 1 Nephi 19:10; Alma 33:15; 34:7; Helaman 8:20; and 3 Nephi 10:16.
(3) Neum, who is mentioned once in the Book of Mormon in 1 Nephi 19:10.
(4) Ezias, who is mentioned once in the Book of Mormon in Helaman 8:20.
The reference in Helaman 8:19-20 to three of these prophets seems to indicate they lived sometime between the "days of Abraham" and the time of Isaiah. [Daniel H. Ludlow, A Companion to Your Study of the Book of Mormon, p. 244]