Demihuman

Demihumans aren't exactly a profession per se, being more of an umbrella term for all human-derived species of the Powers. However, their existence doubles as a way of life in a way not dissimilar to a profession. As a result, they'll be described in that way. Especially as to many of them, being what they are de facto counts as a way of earning a living on their own. As a result, it's fitting to put it that way.
In a significantly less positive note, it's not uncommon for becoming a demihuman to lock you into living in exactly a single way - namely the one of a demihuman.

Career

Career Progression

Their growth in power is a bit harder to convincingly quantify than with mages. Their way of learning magic is generally similar to how it works among mages, however there is a bit of a twist. Demihumans generally avoid side-effects of magic (and learning of magic), which would make them naturally much stronger than human mages. Much, much stronger.
Instead, the most important issue with demihumans is their inborn quality. To compare it to something easier to imagine, how strong a demihuman can be depends on how interested their Power was in them when they were going demi. If they were put together rather haphazardly, you can expect them to hit a bottleneck of spells of relatively low level. They'll still have a notable edge over mages of similar level. But without that limit, a single potent magical library could potentially elevate an entire demihuman community to near-magister levels.
Those quality levels can be inherited. However it's also possible to be born with a varied quality level, typically low-to-mid.
There are exceptions to the rule. There are cases of demihumans transcending their limits, which is believed to be caused by - in a way - them 'going demi' once more and attracting enough attention from the Power to apply some fixes to their work. This is also a necessity if one wishes to become a Nightmare.
This is generally believed to happen through a certain form of mental focus, in which becoming known to many demihumans of your own subspecies makes you more visible to the Power over them. As a result, it's relatively common for heads of larger demihuman communities to receive a rework and become Nightmares. However, there are more factors in play, and sometimes Powers fail to differentiate forms of focus. It's entirely possible to receive a power-up or even a Nightmare update while being universally reviled by your kind.

Other Benefits

Some Initiates actively wish to become Demihumans in spite of how Mages would almost certainly treat them with distrust, having decided that the freer casting of magic are worth becoming outcasts to their former peers. This typically happens when an Initiate is either that invested in the domain of the Power the demihuman species is tied to, or when they have little to lose in becoming Demihumans, sometimes both.

Perception

Social Status

Most demihumans are typically treated with distrust. Many of them are naturally divergent from humans in their way of thinking, making humans naturally vary around them. Most of the initiates are trained from early childhood to watch for threats and act accordingly after all - and for most of them, demihumans register as threats.
There are also demihumans that are treated even worse - some, like Ghouls or flawed celestials are basically 'kill on sight' targets for everyone out there, even other demihumans. It all depends on the species in question and, sometimes, on a place where you live. Approach to demihumans as a whole and to certain species in particular can vary greatly between societies out there.
It's still not entirely uncommon for demihumans to achieve certain degree of popular recognition and become integrate in human society. This, once again, depends on a demihuman in question, their species and the part of the world (or even time) you're taking about. In the end, though, most demihumans are integrated not as individuals but as communities.
A well-organized community of a few dozen demihumans will generally be treated as anything but an easy target, with even larger mage families generally not considering the risks to be worth the potential spoils. If such a community forms, you can expect the locals to establish some ground rules for coexistence and leave it at that. This will last until the community proves itself to be a threat, which is when you can expect the local mage families and organizations to come together.

Demographics

Their joint numbers are hard to quantify, especially with many of them being prone to go either completely feral (especially common with beastfolk), hide yourself from the human population (especially common with ghouls) or simply abandon human society and/or move to Elsewheres.
The highest estimation of beastfolk goes all the way up to twenty million, however the numbers are much smaller if you only include those living on Earth and intelligent enough to be counted as demihumans rather than full-blown animals).