Almost every culture or religion has demons or demon-like creatures. ( by demon-like I mean the biblical view of demons.)
Demons have powers and usually use their powers for evil.
Christianity:
Demons are seen as fallen angels.
Their purpose is to make christians walk more difficult and lead them astray from God.
Demons are seen as followers of Satan or his minions.
Some Christians believe that gods, demigods, and creatures of other beliefs were demons who tried to lead people away from God, which they did.
Greek:
Also had many gods.
No "demons" but evil gods, and fierce creatures.
They have a god who reins over the underworld which is very similar to Hell.
Hinduism:
Hinduism is the main religion in India.
Hindus believe in thousands of gods.
They do not have anything they would actually call a "demon".
Some of their gods have demon-like characteristics.
Kali- Goddess of destruction.
Shiva- When angry causes mass destruction.
Durga- Goddess of wrath.
Shani- Lord who becomes angry very quickly
Demon-like figures:
Hades- ruler of underworld
Aeceus- judge of underworld
Area- curses
Thanatus- Demon like winged creature.
Styx- goddess of underworld
these are just a few examples of the many Greek gods of the Underworld and evil.
Conclusion:
Many religions and cultures have different views of demons or evil beings that could be seen as demons in Christian views, and further proves the existance of demons.