The poinsettia is a recent addition to Christmas decorations and symbols, but it's been around for a long time.  The Aztecs considered it a symbol of purity.  They prised it for the curative sap and the red dye they made from it.  Christians thought the star shaped "flower" (actually red leaves) resembled the Star of Bethlehem and adopted it as a Christmas symbol.
    According to a Mexican legend, a little girl wanted to leave a gift at the side of the manger in the church, a common practice in Mexico, but didn't have anything.  She pulled a weed out of the ground and took that into the church.  When she left it by the crèche, some of the leaves turned bright red to show that her gift was accepted since she gave in the true spirit of Christmas.  SInce then, Mexicans have considered the poinsettia a Christmas flower.  It's American  name comes from Joel Roberto Poinsett, an ambassador to Mexico who promoted it.  
    One legend about the poinsettia plant that is false, but somehow persists, it that the plant is poisonous and can kill pets and infants.  This legend started when a child in a hospital in Hawaii was thought to have been poisoned by eating leaves from a poinsettia, but the doctors later found it was not true.  There is a mild toxin in the plant and if your pet eats an entire plant or a child eats several, they will get a serious stomach ache and start vomiting.  That is not likely to happen because the bitter taste prevents the child or pet from eating much of it.  So don't be afraid to decorate with real poinsettias.