Cornucopia is Latin for Horn of Plenty. Today, we are familiar with horn-shaped wicker baskets used for Thanksgiving as a symbol of abundance. For many centuries, cornucopia was a real horn. Sometimes, it was used for drinking and sometimes in rituals, mostly connected with the end of the harvest season.
Cornucopia is made of two Latin words: cornu (horn) and copia (a lot of, plenty). The term can be traced to Greek mythology, but it was called Keras Amaltheias at first. Again, two words: keras for 'horn' and Amaltheias for 'from Amalthea.' Who was Amalthea? Here, opinions differ:
In all cases, we are dealing with a goat, goat's milk, and a hungry baby god in Crete. Zeus was a lively baby, and on one occasion, a horn was broken from a goat's head. (We have at least two versions - either Zeus broke it, or Amalthea broke it.) From then on, the horn provided milk and other food for Zeus. His divine powers made the horn always full, so there was always enough food for him.
(Neptune offering gifts to the city of Venice by Giovanni Battista Tiepolo)
The idea of such a horn of abundance spread all over the known world. We can find similar horns among Nordic tribes. They are popular among people from South Europe. Horns still play a part in old Slavic rituals.
(Svetovit by Aylett Sammes)
Cornucopia became associated with numerous gods in different mythologies. Apart from Zeus, we can find it at Pluto (Hades), Demeter, Chloris, Gaia, and Fortuna. Old harvest festivals often used a cornucopia as a centerpiece of celebrations. The harvest was over, and the winter was coming. Plenty of food should be stored for the harsh months. Cornucopia, filled with fruits, vegetables, and herbs, was a visual proof of abundance. With a cornucopia on the table, there was no need to worry.
There is also a non-mythological explanation of why a horn represents abundance. Wild goats living on Crete start mating at the end of the hot and dry season. The arrival of the rain in the Autumn is a necessary condition for survival. People living there noticed that the mating season of the wild goats, accompanied by spectacular fights among horned males, coincided with the beginning of the rainy season. Horns bring thunder (Zeus is a god of thunder), and thunder means rain. Rain, of course, is necessary for growth, and a lot of rain means abundance. Milk for Zeus and water for people!
(Spring in Seville by Jose Jimenez Aranda)
Numerous festivals all over the world still use cornucopia as one of the elements for celebrations. Now you know why.