In the Book of Mormon, the Brother of Jared is not specifically named. However we have the following account of Joseph Smith:
While residing in Kirtland, Elder Reynold 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.”– The Jaredites,” Juvenile Instructor, 27:282; Also, Improvement Era, 8:705)
If Joseph said this, then what other options are there apart from lying or being delusional? Why should we disbelieve that the Lord revealed it to Joseph?