By contrast, the Christian doctrine of Jesus' virgin birth is certain, Benedict writes in "Jesus of Nazareth: The Infancy Narratives," which examines the details and context of Jesus' birth found in the Gospels.
"There is no mention of animals in the Gospels," Benedict writes, explaining the inclusion of domestic animals in the manger scene may have been inspired by pre-Christian traditions.
But despite the fact that the animals are fictitious, "No one will give up the oxen and the donkey in their Nativity scenes," Benedict writes.
Indeed, animals appear in the Vatican's own Nativity scene in St. Peter's Square each year, British newspaper The Daily Telegraph said in quoting Benedict's book.
The pope insists in the book the Christian doctrine of the virginal conception -- holding that Jesus' mother, Mary, miraculously conceived Jesus while remaining a virgin -- should be trusted as a Christian truth, calling the doctrine an "unequivocal" pillar of Christian belief.
He maintains Jesus' conception was a miracle involving no natural father, no sexual intercourse and no male seed in any form, but instead was brought about by the power of the Holy Spirit.
"The accounts of Matthew and Luke are not myths taken a stage further. They are firmly rooted, in terms of their basic conception, in the biblical tradition of God the Creator and Redeemer," he writes in the chapter titled "Virgin Birth: Myth or Historical Truth?"
The virgin birth has been universally held in the Christian church since the 2nd century.
Benedict also says in the book the popular belief angels sang to the shepherds to proclaim Jesus' birth -- as in the Christmas carol "Hark! The Herald Angels Sing" -- also is not true.
The "heavenly host" of angels simply spoke the words, he writes.
"According to the evangelist, the angels 'said' this," Benedict writes. "But Christianity has always understood that the speech of angels is actually song, in which all the glory of the great joy that they proclaim becomes tangibly present."
The misunderstanding spawned the tradition of Christmas caroling, Benedict says.
Benedict's 144-page new book is the third in the pope's series about Jesus.