"In the beginning,when the Pony tribes first united,they had no protection from the great Dragons, and the fierce Griffins. They were weak to the dark forces of the Fade. And monsters from Tartarus itself plagued them. They had nothing. So, the ancient elders of the tribes casted a spell, using two young fillies in a ritual designed to bind them to a force strong enough to protect them. the spell was a success, but had a side effect. Three creatures emerged from the void. One was a titan of immeasurable size, and his voice seemed to stir the earth around him. The second, a short bipedal entity, who seemed to bleed mana. The third monster was //////////////////////////////// The two fillies were bound to these creatures, or, rather, the creature were bound to the fillies. The fillies became Alicorns due to the immese power transferred to them during the ritual, and the three creatures, became the unstoppable force of nature in Equestria's name."

Book of Legends- Chapter three, Origins of the Alicorns and others