Bill

Bill is the name of Gandalf's cat.

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.