You were right, it's not the same species as the other eggs. So if it's not a dragon, what is it?
And look, it's gotten bigger! Maybe it will reach its full size soon.

Young dragons are relatively defenseless. Their scales don't grow in until right before they mature, and they have almost no strength. Once a hatchling's wings grow in, it marks the beginning of maturation. At this point they are usually left alone by their mother and begin to hunt and survive on their own.