There's a cutscene that explains this, if you re-enter Cara and Grantes's house right after you get the Light Egg and Soren Flute, where Cara asks why the egg never hatched into a Ra-Seru.GoldenPower89 wrote:I guess Horn was taken out before it could hatch?
As it so happens (based on the explanation), only an individual with true compassion toward the world and a desire to go forth and save it can free a Ra-Seru from its egg. Cara had her love for Grantes (something she deems precious and worth living for), but ultimately didn't possess the love for the world as a whole necessary to hatch the Ra-Seru egg. Then again, she didn't ever seem like the type who would journey as a Ra-Seru wearing hero, so maybe that was for the best.