First of all, the disposition of the appendages on the skull. Second, the morphology: the frontal appendages of Prolybitherium females are much more narrower= they do not have the widened basal area that is present in your Progiraffa ossicones. If you check the posterior part of your ossicones you can see the individual base of each one. That's impossible with Prolybitherium. In fact I strongly doubt that Prolybitherium's appendages have any relationship with true ossicones as seen e.g. in the Giraffidae and the Palaeomerycidae. The overall morphology is completely different, just check our work on Prolybitherium: Sánchez, Quiralte, Morales, Azanza & Pickford (2010). I attach it here.