Thank you both Sajen and Vandoo for your kind response.
I tried to look it up in the books, but I find all different significations (ogre face, demon face, stylized old kind of human figure).
But I read recurrent that it probably has the function of instil fear by the enemy (and why than covering this "fear" face sometimes with hair that you don't see it anymore
) So afterall that could be a signification that it was to strengthen the warrior who holds the shield and give him some protecting power???
Exactly what kind of ideas stand at the beginning of the source of these ornaments???