There are many types of Aegis (head of Medusa) depicted on ancient coins, like those from Selge in Pisidia, from Apollonia Pontica, from Pontus Amisos, and on several Roman Republican denarii. In general, these depictions all conform to the ancient concept of Medusa - she was a creature so hideous in appearance that her very glance could petrify the viewer. Homer speaks of only one Gorgon, whose head is represented in the Iliad (v. 741) as fixed in the centre of the aegis of Zeus. In the Odyssey (xi. 633) she is a monster of the under-world. Hesiod increases the number of Gorgons to three- Stheno (the mighty), Euryale (the far-springer) and Medusa (the queen), and makes them the daughters of the sea-god Phorcys and of Keto. Their home is on the farthest side of the western ocean; according to later authorities, in Libya (Hesiod, Theog. 274; Herodotus ii. 91; Pausanias ~i. 21). The Attic tradition, reproduced in Euripides (Ion 1002), regarded the Gorgon as a monster, produced by Gaea to aid her sons the giants against the gods and slain by Athena (the passage is a locus classicus on the aegis of Athena). However the Aegis depicted just below the throat of several early emperors from Nero to Hadrian, on Roman imperial coins, is something different and special. It is unlike most other depictions of the Medusa in that it has the face of a beautiful woman, but still with both wings and serpents attached to the head. These hint that this particular aegis is a representation of the monster of myth and legend, but the manner of depiction represents the myth that Medusa was originally a lovely woman - it was her tragedy that she was foolish enough to compare herself to a goddess (Athena), and suffer the consequences. (See Ovid, Metamorphoses , Books iv and v.) Thus, this particular depiction of the Medusa is something of an object-lesson ... rather like the man who rode in the chariot behind a general in Roman triumphs, whispering in his ear "Remember - thou art a man!" The first such depiction of the Medusa in this form is attributed to the famous Greek sculptor Phidias (or possibly Kresilas) (c.440 BC). An ancient Roman copy of this depiction was preserved by the Rondanini family in Rome and is now known as the Rondanini Medusa. It is now in the Munich Glyptotek. http://homepage.mac.com/cparada/GML/Medusa1.html and http://www.norwichfreeacademy.com/slater_museum/shows/cast/63_rondanini_medu sa.html For a comparison of Medusa in horrible and beautiful forms, please see: http://www.loggia.com/myth/gallerymedusa.html The first depiction of this Rondanini type of Medusa on ancient coins is (as far as I can recall) on a large bronze of Olbia. But it was frequently depicted on bronzes of Seleukos I Nikator from Syria (3210280 BC). SNG Szaer 22-23 (20mm) and 29-30 (15mm) from a mint at Antioch. For examples see: http://www.seleukids.org/OHS2.htm and http://www.wildwinds.com/coins/greece/seleucia/seleukos_I/Lindgren_1751.jpg The first depiction of the Rondanini type of Medusa on Roman coins is on large bronzes of Nero, where it is seen as an ornament at the base of the emperor's throat. http://www.edgarlowen.com/b5710.jpg and http://www.olympusnumismatics.com/coins/5076.htm It continues to appear on both bronze and silver coins Domitian. http://www.olympusnumismatics.com/coins/0425.htm And finally on coins of Hadrian. http://www.dirtyoldcoins.com/chitlins/id/had/had261.jpg After Hadrian, it disappears from the numismatic record, as far as I know. Based on the design and placement of this imperial aegis, as depicted on coins, I'd suggest it was some kind of brooch in the imperial treasury - part of the crown jewels, as it were. And I'd suggest it was depicted on coins as an affirmation of imperial humility ("I may be powerful and dangerous, but don't worry - I realise I'm a man, not a god!"). Of course, that's just a theory - though it nicely fits the evidence. I don't recall an ancient source that mentions this imperial aegis, nor where it came from or what eventually happened to it.
Athena wore the Aegis on her chest, though rarely. I have a picture of a 4th century BC Estruscan mirror reverse which shows the aegis clearly at the exact center of her chest.When Athena wears the Aegis as clothes on coins the Medusa head is not typically visible, we see just the tassles of the aegis, as on this bronze of Phrygia: http://www.coinarchives.com/lotviewer.php?LotID=38581&AucID=41&Lot=724 . On Bactrian coins she sometimes has it draped over her outstretched arm.The Bactrian kings wore the aegis on the shoulder as on this tetradrachm of Menander: http://www.coinarchives.com/lotviewer.php?LotID=42937&AucID=46&Lot=537 and also on their hats, as the spectacular example I showed earlier. These kings were the first humans to wear the aegis as a ceremonal costume, I think.The Romans emporors wore it from Nero to Hadrian as Harvey said. Later they put the Medusa on their shields as on this coin of Probus: http://www.ancientcoinmarket.com/ds/featured/feat66med/1.html. The best reference is probably _Lexicon iconographicum mythologiae classicae_ but I unfortunately don't own a copy as the set costs many thousands of dollars. If someone has it I'm sure we could find other aegis jewelry examples there.For samples of the aegis NOT on clothes, see my web site at http://www.snible.org/coins/aegis2.html. I've published what I believe is the very first representation of an aegis ever on a coin, previously unpublished!