Ace_108 said:
That child is casca's kid, right. In the tower arc, the behelit guy made him like this, so he would suffer like he did because of his appearance.
Uh.... I think you kinda missed the point of that scene.
The Egg of the Perfect World took pity on the dying child and consumed him so they would die together when the Egg eventually would birth Griffith, as his vessel.
As it turns out, the child was absorbed into Griffith unintentionally and now they share the same incarnated body, with the child only gaining control on nights with a full moon, IE, when magic is at its highest.
Griffith has no control over this change.
But yes, to answer your question, it is Guts' child.