In the fictional universe of the Forgotten Realms, Guenhwyvar is a magical black panther from the Astral Plane. Guenhwyvar weighs about 600 lb (272 kg). The panther was created by writer R.A. Salvatore, who named one of his own cats after the panther. Guenhwyvar is called to the Prime Material Plane by an onyx figurine, most recently in the possession of the drow Drizzt Do'Urden.
Guenhwyvar was created by the ranger-wizard Anders Beltgarden. The onyx figurine, referred to in Dungeons & Dragons 3.5 Edition rules as a "Figurine of Wondrous Powers," was originally owned by Josidiah Starym of Myth Drannor, and has also been possessed by the drow mage Masoj of House Hun'ett, whom Drizzt Do'Urden killed in battle. Drizzt then took the figurine from the ground (where Masoj had left it) because he could not bear to see Guenhwyvar used to do evil; while observing Guenhwyvar forced to hunt down deep gnomes, Drizzt perceived emotional damage being done to a creature he admired and viewed as noble. In his time after leaving Menzoberranzan, Drizzt took to summoning Guenhwyvar as often as possible, to help combat the extreme loneliness that Drizzt faced. In this time they became even closer than before, and they share a deep empathy with each other. Guenhwyvar has saved Drizzt's life more than once in their common history. Even though some of Drizzt's companions (especially Wulfgar and Bruenor) were at first suspicious of the panther, it later became an important "team" member. During the Siege of Darkness, Guenhwyvar was nearly destroyed entirely because of the Time of Troubles, in which magic all across Faerûn was in complete chaos. Because of Catti-brie's quick thinking, Guenhwyvar was saved, and once the time of troubles passed things were restored to normal.
Guenhwyvar is very much like Drizzt. The panther accepts him as a master, but loves him as a friend. Guenhwyvar's relationship to Drizzt is unique. The panther can be summoned forth from the Astral plane up to 12 hours out of every 48 hours. Simply spending time on the Prime Material Plane slowly drains the panther's strength. It is possible for Guenhwyvar to stay longer than the limit, or to be summoned back sooner than usual, but it is extremely draining for the panther. If the panther is seriously injured or even if it 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 it can be called forth again. Guenhwvar also has the ability to take someone with her to a different plane of existence. This ability is first shown in The Crystal Shard, when Guenhwvar transported Regis the halfling to the Astral Plane, thus saving him from being crushed as the tower of Creshinibon was destroyed. Also, in Streams of Silver, the ability was viewed again when the Companions of the Hall were attacked by an insubstantial pegasus, which was able to inflict damage, but became insubstantial against any attacks directed towards it. In the fight, the panther took Drizzt to the plane that the pegasus really was at, allowing it to be effectively attacked.
Guenwhyvar is roughly 600 pounds (twice the weight of a normal panther), and is as smart as any person. Guenhwyvar is "ten feet long, at least, with muscled shoulders as high as a tall man's chest."
There is some debate as to the gender of Guenhwyvar. In the earlier stories the panther was normally referred to with the pronoun "it," but R. A. Salvatore often describes "it" with very feminine descriptions.
In some books—most notably The Crystal Shard—Guenhwyvar is occasionally referred to with the pronoun "he," but in more recent Salvatore books (such as The Hunter's Blades Trilogy) and most outside sources, Guenhwyvar is referred to as "she." On average, Guenhwyvar has been referred to by Drizzt, and others that know the panther well, as "she" far more often than "he." In the novels, the male pronoun is applied to Guenhwyvar less than five times in total. In Salvatore's second book Streams Of Silver Drizzt called Guenhwyvar a "she".
It is assumed by some that Guenhwyvar, being from another plane, is in fact genderless, similar to the Dungeons & Dragons deity Corellon Larethian. Like Guenhwyvar, Corellon Larethian is referred to as "he," "she," or "it."
In Sea of Swords, Drizzt and Catti-brie have a "fight", and Guenhwyvar pushes Drizzt to the ground, holding him there. A line in this passage reads, "the woman had whispered something in Guen's ear, and the cat, obviously gender loyal...", suggesting Guenhwyvar is female.
Also in Siege of Darkness, from the Legacy of the Drow series, it states, "Then for no other reason than to give praise to the day, Guenhwyvar reared and stood tall in the air, a sight that sent her friends' hearts racing.". The sentence structure denotes "her" as Guenhwyvar, with the friends in reference being Drizzt and Catti-brie.
References: Forgotten Realms Setting:Legacy of the Drow Series Written by R.A. Salvatore bring many mentions of the famed panther from the astral plane.