The Sphinx is a feline creature found in Sophanem. She is involved in several quests involving cats.
As the Sphinx puts it, "I am an agent of neither good nor evil. I am unconcerned with the plight of mankind and their petty wars and beliefs." This is likely because the Sphinx is the guardian of feline kind.
The characteristics of the Sphinx are a blend of Egyptian and Greek mythology. In the legend of Oedipus the Sphinx would ask passers by riddles and if they answered incorrectly the Sphinx would strangle and consume them.
Before the Sphinx was graphically updated in the 8 July 2009 update, it was one of the oldest remaining models in the game, and actually seemed to be a male.
In the Wise Old Man's house, he has a golden statue of a sphinx on his bookcase. This may mean he too has spoken to the sphinx and received an approval token off of her. When the player asks him about his life, he will mention that he visited Sophanem and purchased the statue as a souvenir. After he is asked if he has spoken to the Sphinx, he replies that the Sphinx is simply a tall tale and does not exist, though the player will protest that it does. This can only happen after one has completed Icthlarin's Little Helper.
If you talk to the Sphinx after Icthlarin's Little Helper, with a cat with you, and select "I need help with the devourer", she will eventually say that she was impressed with your actions and says that she may have work for you at a later date, meaning that there may be another cat oriented quest.
The Sphinx tells the player when asked about Icthlarin that he once was worshipped by Mahjarrat, but they left him for Zamorak, although she then says it may have actually been Zaros but it was so long ago that it doesn't really matter.