Here's an answer I gave for another naming question. Here's a summary riffing off that: Make yourself three lists 1) the qualities these others have 2) a list of all the words that describe their duties and/or roles and 3) a list of all the qualities humans see them as having. USE only one word each, not a paragraph in this list. Then, hie thee to google with your list and enter each word along with "synonym." You can really use any word and turn it into a name as long as it fits, and doesn't sound stupid. You will come up with many, many options.
It's very easy to complicate something like this, and sticking to one-word description lists, gives you a jumping off point and makes it less complex for you. You're working in generalities to describe a large group of probably very different peoples and that's what's making it difficult. List those generalities/characteristics whatever they are that apply to ALL of them, and you have your start.
If you look at the real world Racists don't tend to have catch-all terms. They like to get specific.
For instance, white folk have been oppressing other groups for centuries, but we don't really have a catch-all term that's popular for non-whites.
Instead, they get specific. There's a whole passel of very specific terms for EACH race.
But I am having trouble figuring out what the characteristics are from what you've written.
Humans were created when, centuries later, a selection of greedy witches decided they wanted more power, and attempted to find the dragon, slaughter it, and take the last of its scales...
What I've tried to do is divine a word referencing scales, snakes, or dragons - something that could have evolved into a slur over time.
which sounds like you want a name for humans?
But the question is:
Creating a derogatory term for non-humans in my fantasy novel?
It seems to more be centered around magic vs. non-magic in which case, the thesaurus is your friend. Here's a link to other words for witches and other words for warlocks and other words for magic.
That's a start, but what you do with it is really up to you and is dependant on your world-- blighter is one of the synonyms, having to do with the negative connotation of magic users blighting crops.
In my eyes, NONE of the words that humans use to reference non-humans of the magical variety should have anything to do with scales. It's the HUMANS that come from scales, not the magical races. Right? Humans were created then? I really can't tell who has magic and who doesn't from this... I will edit this answer as it if you edit the actual question for clarity...
If you want non-magical slurs, just start looking at antonyms and build from there.
If you're looking for a new version of muggle--there's plenty already out there. I've seen mundies, normies and the like already.
If magic is essential to humans (like they ALL have it and nobody doesn't) then you might be better off by starting to think about that.
Non-magic users would then be deemed incomplete, lacking, wanting...
So start with those words as a base, and again, thesaurus.
Flawed, incomplete, bare, empty...
You might even say that the lack of magic is evidence that they lack a soul, which is horrible, but soulless could also be a start. Callous, insensate are some of the words I've gotten off soulless.