Well, yes, there is the Natural Appearance Major Quality, but while that's theoretically a RAW-compatible solution, the idea behind the Quality is that you don't look discernibly different from a human, which centaurs most certainly do.
Granted, the -6 really only applies to normal people, and normal people back then were very different from normal people now. This isn't technically RAW, but since the effect isn't supernatural, anybody who can recognize a given creature for what it is has no reason initially to trust it any more or less than its species' reputation would make you expect. A magus, even one who can't recognize that something is a centaur on sight, really has no reason to distrust it especially for not looking perfectly human, given that interaction with such beings is an expected and relatively frequent part of life, and as a very general rule, a magus wouldn't be so ignorant as to assume that humans are inherently more trustworthy than non-humans. (Most magi will place more trust in Divine creatures and less in Infernal creatures, but even that is a function of Realm, not appearance)
The specific definitions of "attack" and "flee from" also need to be considered, because it's not necessarily literal. If a person with faintly glowing blue skin walked down the road, there wouldn't be a stampede of people running one way and another stampede coming forward with weapons to bludgeon the blue-skinned person to death. However, the person would likely be verbally assaulted, and people would go to great lengths to avoid said blue-skinned person and finish interacting with them as soon as possible when such interaction was rendered absolutely necessary, in a sort of socially appropriate form of fleeing.
Think about this with the centaur. If you saw a centaur trotting toward you in the forest, you might not instantly scream and attack it or panic and run. However, most normal people would be extremely wary, staying very clear of the centaur's path if it appeared to simply be passing by and treating it with immense suspicion and fear if it did try to talk to them. Any action that could be perceived as hostile likely would make most people run, and the suggestion of things like "I could mentor you!" would likely be met with (at least internal) responses such as "Why should I trust him? He might eat people! He might turn me into what he is!"
Mmkay. When making PCs and magi, I can pretend with a straight face that the design rules should be mostly hard and fast. When making a creature of a Realm, that goes out the window and the game shouldn't even pretend to be tight, because each type of creature is unique, and there are more things in Heaven and Earth, Horatio...
That said, I do agree with you that a centaur should effectively look very scary to your average peasant. Monstrous Appearance, of course, is easier to get used to than the Gift, but a centaur would have as much trouble making initial contact with humans as a magus with Blatant Gift would. At the very least, he'd have to say "Fear not!"
...note to self, Angels should also have Monstrous Appearance when not in disguise.
Almost every instance in the Bible of angels showing up flat out terrified people unless they were in disguise. It's probably more of a power than a flaw, though.
Actually in ancient mythology Cetaurs were ll very civilized, but at the sme time they represented divide between the base (animal) nature and intellectual (human) nature, and as such they could all be prone towards uncivilized acts of pssion, but they were clearly not barbarians. That came much later, in the Legends of Hercules TV series...
"Like the satyrs, centaurs were notorious for being wild and lusty, overly indulgent drinkers and carousers, given to violence when intoxicated, and generally uncultured delinquents. Chiron, by contrast, was intelligent, civilized and kind, but he was not related directly to the other centaurs. He was known for his knowledge and skill with medicine."
Which is not to say that there can't be exceptions.
Centaurs originally were a rare grouping of Bjornaer who had become Chimeras through their greater mystery. They were teachers and the wardens of esoteric knowledge.
Chimeras can't have human as a component; in fact, centaurs (and mermaids) are explicitly disallowed in the description. Also, while it's technically a Magic Realm option, it's not really a Magic creature option, as chimeras have no Might and would be just as mortal as the magi they transformed from. I don't really see this working out.
A Bjornaer who has achieved the Inner Mystery of the Chimera will permanently become a magic creature upon entering Final Twilight, and has a Magic Might of (5 x Heartbeast) [Ref: Houses of Hermes: Mystery Cults]. So Chimerae (in the sense used by Thespian) certainly do have a Magic Might. However, centaurs are not a valid option for Bjornaer as they stand, for the reasons you mention (human-animal hybrids are not possible).
My take on this: you've got to look at the role of the centaur in your saga. If it is a creature that yearns to be defeated in combat or which acts as a wise mentor, then it is a Faerie; interacting with humans to gain vitality. If it makes no intentional contact with humans, but just exists 'because', then it is magical. I see most of them as faeries (in common with other human-animal hybrids), which also fits with their connection to the Greek gods. For me, Chiron is a great example of how Free Expression can be used to change a faerie's story.
The point was that I was trying to make a PC magic based centaur in the tradition of Chiron, and it didn't seem like the rules would allow it, and even going faerie was problematic because it would have extremely limited ability to actually teach the way Chiron did, taking seasons instead of trading skills that had been bargained for.
The only centaur that was a teacher was Chiron, as far as I can remember. Most of the centaur stories from Greek mythology portray them as fierce antagonists that ride into a village and attempt to steal its women (the centaurs vs. the Lapiths myth). I've often thought they would be good Mythic Europe orc equivalents, fierce, brutish, bent on destruction and plunder. I like the idea of making them Infernal creatures, especially since Dante puts them in his Inferno (nice reference). If I was going to stat up centaurs, I'd make them Magic Humans with the Monstrous Appearance Flaw, as has been suggested. If I was going to stat up Chiron, I'd make him a faerie, since there are specific myths about him.