Throughout their marriage, the Sun Princess and her Consort of Chaos wished badly to have a child. They wanted a little one to love and call their own who'd be a mark of the love that they had rekindled after centuries of strain in their relationship.
Then one day, their wishes came true. Out of nowhere, a young ball of fur appeared in between them one morning. It was not a day old and had both pony and draconequus traits. After the shock of finding such a vulnerable infant in their bed wore off, Discord soon realized that this was their infant, borne from their desire to have a child. Chaos magic is a funny thing, after all.