If you like to play with your players’ encyclopedic knowledge about Pathfinder/D&D, here are some ideas (I actually used most of them at least once). In my experience it’s always refreshing to change a few game clichés, especially at campaigns with 1st level characters. As usual, the idea to make players think and to entertain them, not to backstab their characters at the last moment with dirty tricks.
- Trolls aren't vulnerable to fire. In fact, they’re supernaturally flammable. Place a troll on fire and he'll lit like oil, causing 2d6 fire damage to all creatures and objects at 5 feet. Oh, and trolls on fire enter a berserk rage (like a Barbarian). After approximately an hour the fire will usually burn out, leaving behind a dark and Exhausted (but otherwise intact) troll. Quenching the fire will result in the same condition, although it’s something considerably hard to achieve [This variant is one of my favorite, in part because every encounter between trolls and player characters is so tactically boring].
- Dragons actually change their scales and innate characteristics. Sages aren't still sure if a dragon’s scales change with age or geography. Yes, that means that scale color has nothing to do with alignment. What about metallic dragons? It is speculated that a dragon gains metallic scales after devouring certain types of creatures. Gold scales, for example, are theoretically acquired after the consumption of pure royal blood (a few princess will do, though a clever dragon can manage the change with a true emperor).
- Lycanthropy is actually genetic. It is passed through generations, not bites. However, wounds inflicted by a lycanthrope induce madness and recurrent episodes of uncontrollable animal rage on the victim. This curse, as far as it’s known, is incurable [No, I never used this once, but I’m totally stealing this from the Malazan Book of the Fallen – by the way, if you liked it, you can go a step further and make lycanthropes immortals].
- Rats (or cats, or wolves or another common animal) are actually magical beasts with Intelligence 10 (just like normal humans). They have their own languages, cultures, magic and societies. Beware: they don’t like their secret being known by “Big Folk”. [I use this option with cats most of the time… blame Lovecraft and my first pet].
- Shadows aren't undead but unfettered shadow of living creatures. When a Shadow drains you all way down to 0 Strength, you don't die, but in 1d4 rounds your shadow rises free as a new monster. “Shadow-less” people have Charisma 3 and are always Fatigued. However, they're ignored by mindless undead and normal animals (and rumors hint to some resistance against Divinations).
- Goblinoids are actually a fungal life-form. They have all the advantages of a Plant type. Their breeding chambers are a series of weird underground (and heavily protected) gardens. These gardens require humanoid blood and flesh to grow and are really hard to eradicate (dragonfire is a safe solution). Oh, remember that plants are immune to poison in Pathfinder. Guess what goblins love to use? [Yup, this is from Warhammer. Besides the fact that it presents a solution to the whole “goblin babies” conundrum].