These are audad skulls or skull plates with or without horns. They have been exposed to the elements and have a weathered look.
Audad, also known as Barbary sheep, aoudad, waddan, arui, and arrui, is a species of caprid (goat-antelope) that is native to North Africa and introduced to North America, southern Europe, and elsewhere. Their distinctive horns have a triangular cross section, curve outwards, backwards, and then inwards.
Audads are subject to CITES and are not for export out of the United States.