I think that the title question is pretty self-explanatory. Since Satan was an angel that rebelled against God are his other demons? What happened anyways? And why did God create angels (especially Satan the worst of them all) like them if He knew (which He obviously did) that they would turn against Him? Angels are supposed to be perfect like Jesus.
We don't know when or how certain angels fell. The Bible simply doesn't talk about it, we don't have Satan's "origin story" as it were.
The earliest mention of fallen angels we have are actually in non-canonical 2nd Temple period Jewish works. The term 2nd Temple period refers to the time from the rebuilding of Solomon's Temple around 516 BC, and its destruction at the hands of the Romans in 70 AD. A lot of Jewish writings were composed during this time, and most of the works written in this time were never accepted as Scripture, either by Jews or later by Christians--but they are nevertheless helpful in our understanding of the sorts of ideas of the time.
It is in these works that we read stories of angels who rebel against God, for example a group of angels known as "the Watchers" are mentioned in the book of Enoch (which isn't a single book, but actually several books edited together), in which these angels end up abandoning their position, have intercourse with human women and who in turn give birth to a race of giants. This is how the book of Enoch attempts to expand upon a very brief and confusing passage in Genesis 6. It also mentions that these angels went on to teach humankind heavenly secrets and mysteries which man was forbidden to know.
Now, the point isn't that any of this is actually true--that there were angels that had sex with human women and gave birth to giants. That simply isn't how mainstream Judaism or Christianity understands the passage in Genesis 6. But what is important here is that this is the time when we start getting the idea of fallen angels.
A common fallen angel from the literature of this time is Samael ("Venom of God" in Hebrew), mentioned as one of the fallen watcher angels in Enoch, and in the Apocalypse of Baruch is a fallen angel who is responsible for placing the tree of knowledge in the garden of Eden, which gets him banished by God, to take revenge he takes on the form of a serpent to tempt Eve to eat from the tree. Samael is the chief satan in the 2nd Temple period literature, I say "chief satan" because in Hebrew "satan" or rather
shaytan simply means "accuser", and isn't always a single individual. The satans, or what we might call the devils (the word "devil" comes from a Greek translation of
shaytan, it means "accuser"), aka the demons. Samael is, therefore, what we might call the common name for the devil in the 2nd Temple period.
By the time we get to the time of the New Testament this idea of fallen angels is already entrenched in popular Judaism, and so the New Testament never attempts to explain the existence of fallen angels/devils/demons (they are all the same thing), but rather assumes their existence. Because Christianity arose out of 2nd Temple period Judaism.
Judaism, after the fall of the Temple in 70 AD would, under the rabbis, come to reject the idea of fallen angels, and would largely ignore angels for the most part, and so the idea of the devil or of fallen angels isn't present in modern Judaism. But it is in Christianity.
So, the short answer is that yes, demons/devils are fallen angels. There is no definitive teaching in the Bible about when or how they fell, only that they did. Literature from the 2nd Temple period talks about fallen angels, and provides a number of stories about them, but none of them are authoritative and shouldn't be taken as authoritative.
-CryptoLutheran
