thanks for the weight and diameter. This is a bit smaller than I was expecting but it does help to narrow the type down a bit further. Basically I still think this is Armorican, specifically a type of coin known as the 'petit billon armoricain'. These are a poorly understood group of coins, very light and in base silver, of many different designs including several with the human-headed horse characteristic of Armorican Iron Age coinage. They are usually found in Brittany and Normandy at temple sites, and they may somehow be equivalent to the silver minims that are found at temple sites in southern Britain. They are thought to be quite late in date, possibly after the Gallic War in some cases (so say c. 40 BC).
Above is his answer. Where was it found, France, Germany, UK?