As of 2015, the writer of the poem "Rainbow Bridge" is unknown. According to About.com, the poem was based on the Norse myths about Bifrost, a bridge used by the gods to travel between the heavens and earth and by exceptional warriors on their spiritual journey to Valhalla.
Three people claim to have written "Rainbow Bridge." One of them is Paul C. Dahm, a grief counselor in Oregon. He claims to have written the poem in 1981, to have copyrighted it in 1994 and to have included it in his book "The Rainbow Bridge," which was published in 1998. The second person is William N. Britton, whose book "Legend of Rainbow Bridge" was published in 1994. The third claimant is Dr. Wallace Sife, whose version of the poem is titled "All Pets Go to Heaven" and featured on the Association for Pet Loss and Bereavement website.