"Probably Charles V of Austria or Carlos I of Spain " (see related A)
Hereditary prognathism (under-bite) is a well established sign (autosomal dominant mutation) characteristic present in about 50% of male or female descendants of Habsburg family members who had this anomaly. The "founder" of this hereditary pattern of facial anomalies (prognathism is the main but there are other alterations) is unknown - what is obvious is that Maximillian I, founder of the Habsburg dynasty inherited this disorder from his father.
For clinical perspective see other entries under Habsburg.
P.S. We are trying to determine who the artist is and where this portrait displayed.