What are the chances of getting a unicorn in the Retired Egg?

Could you elaborate on the chances of acquiring a unicorn from the Retired Egg? I'm particularly interested in the rarity and probability of encountering such a unique creature within this particular egg type. Additionally, are there any strategies or methods that players can utilize to potentially increase their odds of obtaining a unicorn? Understanding these factors would greatly enhance my overall knowledge and experience with this particular aspect of the game.