Guenhwyvar is
Drizzt Do'Urden's magical
panther. She exists in two forms: as an
Onyx figurine and an oversized
black panther.
She was formerly owned by Masoj, a sorcerer from the house of Hun'ett, but later followed Drizzt when Masoj was defeated.
Guenhwyvar (also known as Guenh) is Drizzt's best friend and lives in the Astral Plane when she is in her Onyx form. When she is in the Astral Plane, she is able to conjure multiple forms of herself.
She can only be summoned a few times a day as she must rest in the Astral Plane to regenerate her energy. Although she seems to be almost invincible because of her regenerative powers, she can be thrown back to the Astral Plane by a whistle, notably Jarlaxle's whistle given to him by Gromph Baenre.
Drizzt's famous phrase to call Guenh from the Astral Plane is: "Come to me, my shadow."
Update
I just found out from a website that Guenhwyvar is also the Celtic precursor of Guinevere from the Arthurian Legends. There is evidence that the Arthurian legends were actually based on an old Celtic myth and Guenhwyvar is the wife of Artaius (King Arthur), a Celtic God.
Also, the Gaelic pronounciation of Guenhwyvar is GWIN-hwee-var.
http://www.cybercomm.net/~grandpa/celtic.html
See also: Dark Elf Trilogy and Drizzt Do'Urden