There were many OTHER GODS besides the Olympians. A number of them were said to live atop Mount Olympus with the supreme twelve.
For instance, Heracles was made into a god when he died, and he was given the goddess Hebe as his wife. They lived together in a palace on Olympus. Only the Olympians themselves, however, met in supreme council, seated together in Zeus's royal hall on twelve ornate thrones.
For that matter, the composition of the Olympians changed over time. Originally Hestia, goddess of the hearth, was one of the twelve, but she grew tired of the godly bickering and gladly gave her place to the god of wine, Dionysus.
On the other hand, Hades, god of the dead and brother of Zeus and Poseidon, isn't included in our assembly because he dwelt in the Underworld and not on Olympus. His Roman name was Pluto.
Pan (who is pictured above in the act of sneaking into a gathering of the twelve Olympians) was a god of shepherds and flocks. Pan was the son of Hermes and a nymph. He was born with the legs and horns of a goat, which caused his own mother to spurn him.
Nor was the adult god more popular with the nymphs. Echo ran away from him and lost her voice as a consequence, being condemned only to repeat the words of others. Another fleeing nymph was transformed into a reed, which inspired Pan to invent the shepherd's pipe of bound reeds of varying lengths.
Pan was considered to be the cause of the sudden fear that sometimes comes for no reason, especially in lonely places. That's why it's called "panic".