Being the bastard spawn of a drunken orgy between Dungeons & Dragons, Hammer Horror films and Gothic Horror novels, it should be no surprise that the setting of Ravenloft is full of murderous monsters that look almost, but not quite, human. It should be equally unsurprising that it is also full of superstitious, xenophobic, racist peasant-folk, although how much the former informs the latter is a matter of personal taste. Needless to say, if you're not playing a human, chances are the NPCs aren't going to be too friendly.
When the Ravenloft campaign setting was released for Dungeons & Dragons 3rd Edition by White Wolf, this was handled by the introduction of a new game mechanic: Outcast Rating. A simple little numeral that represented just how much you freaked out the local ignorant peasants. Whilst mostly static, there were some ways of raising it - certain curses or failing Powers Checks would boost it, and some monsters could also inflict it upon you; goblyns had the "Feasting" attack, which left such horrific scarring from their attempts to eat your living face off that you permanently gained Outcast Rating as a result. And, of course, DMs had carte blanche to inflict it upon you for various "thematically appropriate" reasons.
Mechanically, Outcast Rating manifests as a Circumstance Penalty to Bluff, Diplomacy, Gather Information and Perform checks, but with an equivalent Circumstance Bonus to Intimidate checks. Outcast Rating has no effect on friends & allies (who have learned to look past whatever makes you spooky), nor on beings who share your Outcast Rating "origins" - you should be able to figure out why this is, so long as you have a better brain than a moldy half-cabbage.
Of the "core" Ravenloft races, their respective Outcast Ratings were:
- Caliban: 5
- Dwarf: 3
- Elf: 3
- Gnome: 2
- Half-Elf: 1
- Halfling: 1
- Half-Vistani: 2, but can attempt a Disguise Check (minor details DC) to reduce to 0
Of course, with the long tradition of Outlanders (aka, PCs from beyond the Demiplane of Dread in Ravenloft and the even longer tradition of people playing fantastical races in D&D, this mechanic needed a little "oomph" beyond just informing players what the standard races got. Enter the Ravenloft Dungeon Master's Guide for 3.5, which gave DMs the following rules for establishing Outcast Ratings.
Firstly, check Size. Medium creatures gain +1 OR to begin with, Large ones get a +2, and Huge ones get a +4, with the implication of a further +2 for each size category beyond that. Small creatures have a +0 OR, and Tiny ones a -1 OR. Basically, the bigger you are, the more obviously threatening you are.
Secondly, check Appearance. Creatures with a Slightly Inhuman appearance get +1. Those with a Distinctly Inhuman appearance get +2. And those with flat-out Monstrous appearance get +3. There's a lot of wriggle room on what appearances count as what. For example, a goblin is cited as an example of a Monstrous Appearance, but what if you're playing one of the non-hideous goblins that came out after AD&D? Expect monstergirls players to really argue about this aspect.
Thirdly, check Reputation. Basically, what is your racial stereotype amongst humans? Angelic races, being the goodest of good guys, get a -2 modifier to their OR. Simply Good races get a -1 modifier. Then you have the negatively stereotyped races, consisting of Evil (+1), Predatory (+2) and Fiendish (+3) - a Predatory race is characterized as not only evil, but actively menacing to humans; calibans are looked down upon as ugly, brutish and dumb, but nobody believes they inherently want to hurt "normals", so they're only an "evil" race, whereas a dhampir or ghul, if mistaken for their vampire and ghoul progenitors, would be a Predatory race. A Fiendish race, in comparison, is the absolute worst of the worst, with the book itself saying this Reputation is mostly associated with fiends - so a very obvious tiefling or a shadar-kai might get slapped with this.
Though not discussed in the book, Reputation is one area where the player can try to make the more Low Fantasy focus of Ravenloft and its planar isolation actually work for them. It's not explicitly said, but if you don't look a creature that the average mistlander recognizes or knows anything about, then presumably that's a +0 to your OR. So, if the local yokels don't recognize you, that might not impact you, or even benefit you because of how little they know. After all, with no native drow or duergar, there's no local legends about them, so it's actually plausible that they don't recognize you as an Evil race - if anything, they might regard you as just a normal elf or dwarf, which means you actually get mistaken for a Good race and so drop your OR by -1!
Lastly, check for Other Factors. These are a grab-bag of random things that might impede your reception by Ravenloft's backwater people. Firstly, there's the Beyond the Pale factor, which basically boils down to "if you're not a Humanoid, Monstrous Humanoid, Beast or Magical Beast, add +1 to your OR". Secondly, Culture Shock means you might get a +1 OR if your native culture is really different to the culture you're interacting with, or if there's a historical friction between your cultures (for example, nobody likes Falkovnians). If you have Unnatural Powers, that's a +1 OR. Finally, at the DM's discretion, being particularly gruesomely scarred, mangled or visibly injured may boost your Outcast Rating by an amount corresponding to how hideously maimed you are.