Medusa skins are a skin family that currently has only one member, it being the AWP | Medusa. The skin features a unique paint job depicting an image of the Gorgon on the side of the weapon. AWP Medusa was added to the game in 2015 during the Operation Bloodhound update and is a part of The Gods and Monsters Collection. The weapon is available in all five exteriors, however, it can't have StatTrak attached to it.
In terms of value, the skin is considered to be extremely rare and goes for upwards of $2000 depending on the exterior and quality.
Check out other CS:GO skins families: