Who is Beelzebub/Belzebu?
The word Beelzebul (Βεελζεβούλ in Greek) (in Hebrew: בַּעַל זְבוּב), means: lord of the flies; this word comes from the Hebrew language: Ba’al Zebub (בעל זבוב). Beelzebub (name derived from Baal Zebul or Baalzebub, also referred to by Belzebuth. In fact, this deity was worshiped by the Philistines in the biblical city of Ekron (עֶקְרוֹן), about 30 km from Jerusalem.
The Canaanites deposited perishable offerings to what they called Lord of Hosts, with time, they rotted on the altars on which they were placed, so the Jews began to mock this foreign deity, giving it the name "lord of the flies", because these insects took care of putrid offerings. Ba'al, in fact, also means lord, in Hebrew. But no one could worship any deity other than IHWH (Yahweh), much less if Ba'al was that deity.
Is Beelzebub a demon?
In the New Testament, the name Belzebul - in Greek Beelzeboul - was used as an adjective for Satan, which means "prince of demons" (Matthew 12:24, Matthew 10:25 and Matthew 9:34).
The Bible mentions the names of many false gods, but in reality they are not demons. These gods were built by men and lead man himself into deception (Jeremiah 16:20, Isaiah 43:10, Psalms 96:5).
Attention: Everything that does not come from the Truth comes from the lie and the Devil is the father of the lie (John 8:44). Satan takes advantage of the name of false gods and religions - created by men - to spread deception, idolatry and death. In this way, it manifests itself on behalf of these entities with the purpose of promoting deceit and destruction (1 Peter 5:8 and Matthew 4:8-10).
Text Beelzebub / Belzebul in the Bible, Matthew 12:22-32
Then a blind and dumb demoniac was brought to him, and he healed him, so that the dumb man spoke and saw. 23 And all the people were amazed, and said, "Can this be the Son of David?" 24 But when the Pharisees heard it they said, "It is only by Be-el'zebul, the prince of demons, that this man casts out demons." 25 Knowing their thoughts, he said to them, "Every kingdom divided against itself is laid waste, and no city or house divided against itself will stand; 26 and if Satan casts out Satan, he is divided against himself; how then will his kingdom stand? 27 And if I cast out demons by Be-el'zebul, by whom do your sons cast them out? Therefore they shall be your judges. 28 But if it is by the Spirit of God that I cast out demons, then the kingdom of God has come upon you. 29 Or how can one enter a strong man's house and plunder his goods, unless he first binds the strong man? Then indeed he may plunder his house. 30 He who is not with me is against me, and he who does not gather with me scatters. 31 Therefore I tell you, every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven. 32 And whoever says a word against the Son of man will be forgiven; but whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come.