Okay, this is all subjective, it's not the be all end all, it's just how I interpret it.
Besides, maybe Kevak has a more comprehensive theory about it (then again, maybe not).
The different areas of the world have different origin myths for their supernatural creatures, but without absolute knowledge of which contradicting story is true, we'll go by a simpler one: Supernaturals have been around for most of Human history. If we're an offshoot of the supernaturals, the other way around, or entirely different species, is unclear.
First let's turn our gaze to Europe, which we'll split into several areas. The Celtic, the Germanic, and the Greek.
The Celtic is mostly around the British Isles, the creatures here often end up in the category called Fae.
The Germanic is generally around Germany, but spreads far, they have Fae, but also the Ab-Human, with a fair share of Beasts.
The Greek have almost no Fae in comparison, they have a lot of Ab-Humans, but they do have Beasts.
Generally there's at least one of each kind of supernatural in an area.
The Fae are natural spirits, they appear in places of plentiful natural magical energy, but can become hostile if their environment is threatened or desecrated.
Malicious Fae can often be warded off by cold iron, kinder Fae seems unaffected by this, and is generally best to treat kindly, as their aid can save your life.
The Ab-Humans are creatures very similar to Humans, like giants, dwarves, jotuns, minotaurs, etc. These have a Human element to them, and aren't as closely linked to magic.
Ab-Humans don't seem to have any specific weaknesses, but they're also mortal creatures, and subject to most of the same rules Humans are.
The Beasts are creatures of instinct, most aren't even sapient, and they tend to be big and scary. Beasts generally have 1 or 2 powers, like regeneration or elemental abilities.
The most dangerous of the beasts are the Dragons, who combine strength, intelligence, and magic.
The magic used in Europe falls into 3 categories: Arcane, Primal, and Elemental.
Arcane is the most difficult to use, and mostly used by Humans. Arcane uses the Arcane plane, a realm beyond the physical one, and if you're not careful, you can let through more than just energy. It's recommended to suppress emotions when dealing with the Arcane, as it can serve to distract you. Most Arcane spells start orderly, but more experiences (or talented) users are known to personalize their spells, to show off.
Primal is based on the life around you, and possibly within you, but that's rather dangerous. Primal magic is much safer, but relies on you gathering power from places of much power. Primal magic is often channeled and/or fueled by emotion, and it's therefore detrimental to suppress emotion when dealing with it. Primal users can gather power from sacred groves, spirits, emotions, and even drain the life of others. Though the last is generally looked down upon.
Elemental uses the powers of Fire, Wind, Earth, and Water. These powers can be combined to make new elements. Elemental is considered to be between Arcane and Primal, as it's channeled by emotions, but also so dangerous you shouldn't be careless about it.