Medusa first appear within the Dark Palace. They are stationary stone statues that will occasionally shoot fireballs out in Link's direction. It is common for there to be multiple Medusas in a single room, usually positioned in the corners. However, sometimes they are found in clusters in the middle of rooms or individually in random rooms.
Medusa, like Stone Statues from other Zelda titles are more along the lines of traps as opposed to classical enemies. They cannot be harmed with any of Link's weapons. If Link is hit by a fireball, it will do a full heart of damage. The fireballs can be deflected using Link's Fire Shield. If Link still has a Fighter's Shield, then he can only hope to avoid the attacks.
Screenshot of Medusa from the Dark Palace.