The Pentagram Coin was shipped with the original release of Ultima VIII: Pagan as a trinket. It is a real metal coin with a pentagram engraved upon it. The coin itself does not have an in-game equivalent like many other trinkets, but the most likely explanation is that it is one of the symbols of Sorcery, which look much like the Pentagram Coin, but are black.
There are numerous pentagrams present throughout Ultima VIII. The pentagram represents an item of magical power and is used for the binding of sorcery spells. The pentagram is also present on the original front cover of the Ultima VIII box (later releases did not have it), and caused much controversy during the release of the game.