LUKE 10:18 "And Jesus said unto them; I beheld Satan as lightening fall from Heaven."
There are millions and millions of spirits, both in Heaven and on earth. It is fact they cannot be seen with physical eyes, but God has given them power and ability to go and be anywhere they want to be. God does not force spirits to serve Him, He gives freedom of choice to all beings. Spirits do not have a body of flesh, but they can enter and live in any flesh body of people or animals. The spirits in heaven that are faithful to desire a relationship with God and serve Him become, "Angels" and are used to do God's work. This will be too short to print and permit some to see as they should; but the eternal truth can be given to be a greatly needed message, God wants to give. So consider the truth; the only way you can be free from evil spirits is to know you have God's Holy Spirit within your living. God is supreme over all creations, and His Spirit is the strongest spirit to be known, He can keep all evil spirits out of your life and decisions. But Satan was created as a beautiful angel and was given great power that he still has. He was also given freedom of choice to choose; but he became such an evil spirit he is God's enemy. Please know he is your enemy too.
Think about the beneficial story of Legion in your Bible. He had so many evil spirits in himself that he had no friends and was lonely. He was brought to Jesus's attention and Jesus came to heal him. All spirits of any kind know Jesus and are afraid of Him. The evil spirits in Legion knew Jesus would cast them out, and they asked to be put into a herd of swine that was nearby. Jesus cast them into the swine and the swine ran down into the sea and were drowned. So yes, spirits can live in people and animals also. Some animals are mean and evil and are a danger to people, while others love people enough to step between danger to people and take the hurt themselves.