Below is taken from Wikipedia.
Guenhwyvar is a magical black panther called from the Astral Plane by an onyx figurine, most recently in the possession of Drizzt Do'Urden. She is described as "ten feet long, at least, with muscled shoulders as high as a tall man's chest." Guenhwyvar is the high elvish word for shadow.[25] Guenhwyvar is the creation of Anders Beltgarden.[25] The onyx figurine is originally owned by Josidiah Starym, and has also been possessed by the Masoj of House Hun'ett, who is killed by Drizzt in battle. He does not wish for Guenhwyvar to be used for evil purposes, as it causes emotional damage. He summons Guenhwyvar as often as possible, to help combat loneliness, and to help him in battle. His companions, notably Wulfgar and Bruenor, are often suspicious of Guenhwyvar at first, though they eventually come to respect Guenhwyvar as a team member.
Guenhwyvar can be summoned forth from the Astral plane up to twelve hours out of every forty-eight. Simply spending time on the Prime Material Plane slowly drains the panther's strength, but it is possible to ignore the limit at the cost of serious injury and pain. If the panther is seriously injured or even if she suffers injuries that should be fatal while on the Prime Material Plane, the panther transports back to the Astral plane. Spending a sufficient amount of time there regenerates the cat completely and she can be called forth again.
She is friends with many characters, including all of the Companions of The Hall, however she is closest to Drizzt, Cattie-Brie and Regis. She's called Guen by Cattie-Brie, and has been sent to sleep on both Bruenor and Belwar Dissengulp.
In the Icewind Dale trilogy Guenhwyvar is portrayed as a male. In The Legend of Drizzt series (printed after the Icewind Dale series) and to Starless Night the gender of Guenhwyvar is not touched upon. Later books portray Guenhwyvar as a female.