Part of the brother of Jared’s name appears in the Book of Mormon, they called the name of the place Moriancumer (Ether 2:13), but no connection is made with the brother of Jared in that verse. The prophet Joseph Smith revealed the name in the following manner:
“While residing in Kirtland Elder Reynolds Cahoon had a son born to him. One day when President Joseph Smith was passing his door he called the Prophet in and asked him to bless and name the baby. Joseph did so and gave the boy the name of Mahonri Moriancumer. When he had finished the blessing he laid the child on the bed, and turning to Elder Cahoon he said, ’The name I have given your son is the name of the brother of Jared; the Lord has just shown (or revealed) it to me.’ Elder William F. Cahoon, who was standing near, heard the Prophet make this statement to his father; and this was the first time the name of the brother of Jared was known in the Church in this dispensation.” (“The Jaredites,” Juvenile Instructor, 1 May 1892, p. 282)
The relationship of Mahonri with Jared is equally interesting. Jared may have been the elder brother, for the recorded genealogy went through him, but, clearly, the spiritual leader of the two was Mahonri. Jared continually goes to his brother whenever they needed something from the Lord.
Jeffrey R. Holland
“One of the greatest prophets in the Book of Mormon goes unnamed in the record that documents his remarkable life. He is identified only as ’the brother of Jared.’ Yet the revelation that unfolded before his eyes was so extraordinary that his life and legacy have become synonymous with bold, consummate, perfect faith.” (Christ And The New Covenant, p. 14 – 15)