I have to agree with previous statements... They usually are either direct copy-pastes of the (Greek) myth
(A winged lioness with the face of a woman, who guards a location and eats anyone who fails to solve her riddle) or a monstergirl with varying types of slightly standoffish but loving personality.
DnD has a notable addition in making them covet knowledge, wit, and treasure... but that's probably purely a justification for the riddling and the phat stacks of cash you can get from killing them.
The Egyptian Sphinx was a symbol of the Pharaoh's power, the sun, and protection. Being a guardian, highly intelligent, and ferociously strong are the overlapping traits of the Ancient depictions of the Sphinx. So those are the traits I would highlight writing one.