Certain entries in the Bestiary contain talents that are unique weapons to those creatures. For example, the Industus carries the Razor Silks weapon.
It is intended that these weapons be considered "guaranteed" loot drops; should the players defeat one of these monsters, and the monster used the weapon throughout the fight, it is only reasonable that the players could retrieve them.
Each of these weapons has a normal weapon entry, and so can be used by player characters. They don't have a value attached to them, although if needed, consider them to each be worth 500 Crowns (due to either their power, or novelty).