Regality
"Keeeeh!"
The egg cracked and out came a chick that had its eyes closed, which he didn't open for some reason. It was coated with amniotic fluid when it first broke free of its shell. It didn't have any feathers over its body.
The chick had its beak open because it was crying at Roo who was now sitting over his master's shoulder. He had stopped trying to scare Eren. It was his time to be subdued by a beast of a higher tier than him.
Eren had been taking care of the egg in his free time just as instructed by Layla ever since he came out of Purgatory. He would often imbue it with a semblance of his mana in a controlled manner. Then he would wrap his mana sense around the egg to make the embryo get attuned with his mana sense.
Apart from safeguarding the egg, the runes over the egg were also meant to act as the medium between the beast and its tamer. They allowed the beast to get familiar with Eren's Ace Rank mana straight away while it was still in its pre-birth state.