Bill is the name of Gandalf's cat. In December 2004 she was rescued from the RSPCA kitten shelter. Her exact breed is unknown, hoever what is known is that her tail is unusually fluffy compared to the rest of her fur.
Origins of the name
Because of Gandalf's short attention span, he saw fit to name his cat after several different personalities, both real and fictional:
Bill S Preston Esq, from Bill and Ted's Excellent Adventure.
Bill Clinton, US president.
Bill, the main villain in Quentin Tarantino's Kill Bill.
William "Bill" Shatner, known for his portrayal of the divine Captain Kirk.
Officially, Bill is short for Wilhelmina, as it is the only female name that can be shortened to Bill.
Bill's hobbies include sleeping on the couch, sleeping on piles of clothes, and taunting the dogs that live nearby.