It depends if you're wanting to be non-religious fantasy or a religious based fantasy. (AKA, based off popular views of the religious entities in question, or the actual old religious stories about them.)
Not Religious Fantasy
You can have it simply be different breeds or subspecies. Many species vary wildly across individuals (think queen bee vs worker bee or Great Dane vs pocket poodle.)
However, if you're going religiously based, angels and demons are not directly related. Biblically speaking, they're two very different creatures (possibly different species) and nowhere in the bible does it claim they're the same. It does, however, imply that fallen angels join demons to fight along side them (although, interestingly, no demons every rise to side with God).
Before the Bible was consolidated into one book, the origin of demons was made very clear. All demons are specifically the children of Adam and Lilith, as opposed to humans who are the children of Adam and Eve. You'll notice in the bible there are two creation stories: First is the 7 days story (where god creates man and woman at the same time), and the garden of eden story (where god just puts man in the garden of eden, and then makes women from his rib.) What happened to the first woman was left out when the various works that make the bible were consolidated into one book. The first woman, Lilith, didn't care being told what to do, and refused to have kids with Adam. This angered god, who then chained her up and... well.. god got what he wanted from Adam and Lilith anyway, without her consent. However, Lilith, being so treated, made sure her kids would know exactly what she thought of God.
The children of Lilith were the 'original' humans by design. They had both from-scratch male and female, and were fully human. Eve, on the other hand, was a copy of Adam, made from his rib, and made female. So Adam and Eve's children were only half of what humans were supposed to be. Further, the serpent who tempted Adam and Eve was supposed to be one of Lilith's children (the story about it being the Devil wasn't added in until King James).
This means that Lilith's children managed to get access to both the fruits of knowledge and fruits of life before god kicked everyone out of the garden. And Genesis 3:22 makes pretty clear that's not what he wants and why. So god basically declares war on Lilith and her children.
As they're the "full" humans, fueled with both divine fruits (unlike humans which are only "half" humans with half of the divine fruits), this is where demons got there name.
The "mon" suffix as in "Human" (remember, not originally english)
and the prefix "Dei" as in "Divine". How it was originally pronounced may have been closer to "Diman"
Demon literally means "Divine Humans", and are supposed to be the epitome of what humans could be, and were striving against God as a whole.
Obviously, when putting together the collection of works to make the bible, this makes demons look a better, and god look a lot worse (as forced intimacy and genocide are generally frowned on), so it got conveniently left out. However, if you want dig through all the old Jewish Midrash and more, it's referenced fairly often.
Although Hebrews at the time, when the story of Lilith got left out, still understood Demons and Angels were different species, this made things much later for later generations and Christians who picked up on the text. They tried to explain why there was such animosity between god and demons, and so it was crossed with verses about fallen angels joining demons to try and explain it away. And although it's not in the bible anywhere, that explanation has kept going through Christian tradition till modern times.
Here's a diagram if it helps:
Now, that said, the Bible ALSO says that mankind is made in God's image. If we interpret that a different way than Christians traditionally do (basically meaning instead of a self-portrait, that humans are God's offspring), then demons go right back into being the same species as humans, and in turn, potentially angels and nephilim (and god himself) as well. Considering the ability for all of the above to have offspring with eachother, this actually seems a moderately likely interpretation: That God, Angels, Demons, and Humans are all different breeds of the same species. In this regard, we're back to the different breeds or subspecies being what creates different appearances again.