The egg you chose hatched into a vicious Green. Better feed it before it decides that you taste good.
Greens are generally smaller than any other color, and are almost always female. They make up for their lack of size by being incredibly vicious, but they're the easiest to tame, and will, usually, become unbelievably loyal to one person, and will defend that person to the death. Greens tolerate other Greens well, but tend to be picked on by the larger colors.