Below is a list of the rare weapons in Dark Souls, as well as how to come across them in game:
Cut off the gargoyle's tail to get the Gargoyle Axe.
Dragonking Great Axe:
For this axe, cut off the Gaping Dragon's tail.
The Moonlight Greatsword requires you to remove Seathe the Scaleless' middle tail.
Cut off the tail of the Covenant Leader for Path of the Dragon, the Everlasting Dragon. This won't make him hostile, but you should ensure you actually attack his tail rather than his body.
Remove Drake's tail for this sword.
To get Priscilla's Dagger, you have to cut off Priscilla the Crossbreed's tail.
Added by: MongooseGeneral Jan 7th 2013, ID#8944