The Prophet Joseph Smith taught,

“The sign of the dove was instituted before the creation of the world; a witness for the Holy Ghost, and the devil cannot come in the sign of a dove. The Holy Ghost is a personage, and is in the form of a personage. It does not confine itself to the form of the dove, but in sign of the dove. The Holy Ghost cannot be transformed into a dove; but the sign of a dove was given to John to signify the truth of the deed, as the dove is an emblem or token of truth and innocence.”

The Teachings of the Prophet Joseph Smith, p. 276