Well, the chick grew up into a chicken, but you’re still not sure how the egg got into that cave...

When the egg appeared at the Mint Garden gate, Mentha knew it wasn't a dragon egg.  He called the little dragongrrrl, Merrian, the human child he had raised after she was also abandoned as a baby at the gate.  "I know this isn't a dragon egg, but it is still very special, and I think you might have fun raising it," he told her in the telepathy she had readily learned.  "Thank you, Papa!" the grrrl replied, and she delightedly took the egg. She made a pouch to wear like a necklace under her sweater, so she could keep it warm. The little fluff ball creature which emerged followed her everywhere as it grew. It loved the little dragongrrl, and thought of her as "Mama."

There is much speculation about the origin of the egg.  Some dragons say it came from space, others call it a ninja, or a mutant dragon. Merrian thought it was just super, and when she asked her papa, he told her he believed it was a "chicken." While all these names are used, Merrian just calls her Susie.