So I rotated the logic compared to the lecture: in “front view” I want to have the front of the model visible … For me, that is exactly the reference picture so the mirror needs to run along the XZ-plane (i.e., in the Y direction). I know, in fact then you have the “side” of the horse head, not the front, but as a chess piece, I consider this as front-facing … Not that it matters much here, I guess … Even in general, I often forget to properly align front and right side according to the axes (the top side, I obviously do correct intuitively).
Not the most challenging “challenge” but anyhow 
