Answer:
Greek mythology is a religion, a set of beliefs that the Old Greeks believed in. Like today we have Christianity, Buddhism, Islam, and many other religions, so did the Old Greeks have their own religion, or something we refer to as mythology nowadays.
Their mythology was polytheistic, meaning they believed in many gods such as Zeus, Hera, Hades, Poseidon, etc. and many different beings such as Medusa, Pegasus, Minotaur, etc.