Draft of Endymion by John Keats, c 1818. Endymion is a poem by John Keats first published in 1818. It begins with the line A thing of beauty is a joy for ever. Endymion is written in rhyming couplets in iambic pentameter (also known as heroic couplets). Keats based the poem on the Greek myth of Endymion, the shepherd beloved by the moon goddess Selene.