Here's Phillip II of Spain and he's wearing a gold necklace with some sort of dead animal on his necklace. I can't seem to find any resources explaining what it is or what its significance was, yet it seems to be a prominent accessory in many Habsburg portraits, from the 15th century on.
It is the insignia of the Order of the Golden Fleece, a chivalric order dedicated to Virgin Mary and St. Andrew. It was established in the 15th century by Philip III, Duke of Burgundy, but soon the Habsburgs acquired sovereignty over it. In 1477 France annexed Burgundy proper, but the ducal title and possessions in the Netherlands – and the Order – went to the Habsburgs with Maximilian I marrying Mary, the daughter of the last duke, Charles.
After the house of Habsburg split during the reign of Charles I (or V), the Order passed to the Spanish line. The War of the Spanish Succession then saw both sides claiming the Order, which resulted in its division, with Bourbons in Spain and Habsburgs in Austria both keeping their own branches. Some time later, Philip V also tied the Order to the crown of Spain rather than the title of Burgundy.
After this the two Orders have developed in their own way, for example the Spanish branch even accepted non-catholics, and the Austrian one was in some trouble after the fall of the Habsburg monarchy following the Great War, but in the end both Orders of the Golden Fleece remain to this day.
