The following is from a work-in-progress called The Mahabharata: a Book Report, in which I'm slowly reading through the Hindu epic poem The Mahabharata, and writing a book report on what I read.
Then Saunaka, chief of the seers, asks Ugrasravas to tell the events which led up to King Janamejaya's snake sacrifice, where he heard the Mahabharata, so Ugrasravas tells the following story:
A seer named Kasyapa granted each of his two wives (Kadru and Vinata) a "boon" (a special gift). Kadru asked for a thousand snakes for sons; Vinata asked for two sons of equal strength. So Kadru produced a thousand eggs which, after 500 years, hatched into snakes. Vinata produced two eggs. After 500 years, she became impatient and broke one open. Out came an incompletely-formed bird named Aruna, bird of dawn. Aruna was so angry at his mother for his premature hatching that he cursed her to be a slave to Kadru for 500 years, after which she would be freed by her other son, the divine bird Garuda.
One day, the wives Kadru and Vinata came across the horse Uccaihsravas, who was born from the ocean. At this point, the seer Saunaka interrupts Ugrasravas's story and asks to hear the origin of this magical horse, so Ugrasravas tells it. [Note that this is a digression from a digression in the story. The Mahabharata is full of these. It's a multi-layered narrative.]