Inspired by Nature: Predators Attack

If you watched the Peter Jackson version of King Kong, you'll know this scene. The girl, King Kong and a t-rex are falling. They're caught in vines and they're swinging back and forth. Any minute, the vines will snap and send them tumbling to the ground. The t-rex, as well as falling, is under attack from King Kong. What is on the t-rex's mind? The danger from falling and the giant gorilla would be a good guess. Wrong. All the t-rex wants is to eat the bite-sized girl.

You can laugh at the film all you want, but books do this too. It's especially common in fantasy books. The predator will stop at nothing to kill the heroes, for no apparent reason.

Predators will attack people, but there are things to consider. Many predators are frightened of humans (elves, dwarves, insert sentient race here). This was demonstrated by Nigel Marven, a naturalist, who jumped in the middle of a pride of wild lions (don't try this at home). The lions scattered. People are scary, because predators don't want to get hurt. Injury means death for many wild animals. Even when a predator could subdue a human, there are many easier snacks to hunt.

So why would a predator attack a party of fantasy adventurers?

• Food Scraps: The biggest problem with wolves is not attacking people. They're timid. It's stealing food. They'll follow and wait for a moment of weakness when food is left unattended. This sort of attack isn't immediately dangerous, but could lead to starvation later on. What will the party do when they don't have enough food for everyone?
• Surprise: There are times when sneaking around is a bad idea. Though most predators avoid people, they may not hear them coming. A cornered animal will attack. It will also escape as soon as it can, so the party could diffuse the situation by backing off.
• Unfamiliarity: If a predator doesn't know what a human is, it might try to attack. This is not going to be a continuous attack till death. It's testing the waters. A suitable show of resistance could drive it away. Dominant behaviour, like staring at it or shouting, might cause it to run before it attacks.
• Familiarity: Feral dogs are far more dangerous than wolves. They're so used to humans, they don't see them as a threat. Familiarity could also mean the animal is used to eating humans. Crocodiles are more likely to attack in regions where they've been fed dead bodies. They learn to associate the smell and taste with food. A live human is more dangerous, but crocodiles are not the sharpest tools in the shed.
• Individual Difference: A larger predator might have some early success with attacking humans. That individual could start to seek out humans as prey, despite the fact its species normally wouldn't. Man-eating tigers are a good example. Tigers don't usually eat people, but a few get a taste for it. Due to their strength, they have a good chance of over-powering a lone human. They may manage to pick off a sizable number of people before they're hunted down. Being hunted down is inevitable, and will put a stop to this behaviour becoming more common in the tiger population.
  
  Many dragon stories follow this sort of pattern. It isn't a colony of dragons terrorising the village. It's an individual dragon.
  
  This will affect your world-building. If a man-eater is on the loose, the local villagers would be talking about it. Perhaps the adventurers have been hired to deal with it. It would be odd for a man-eater to pop up out of the blue, with no one else knowing about it.
• Great Size: If your predator is the size of the average cathedral, a few humans aren't a big deal. It won't get hurt. Though would they be worth eating, based on size? It might be like a whale sucking up krill. It could hoover up whole villages.
• Weakened Prey: An injured and helpless human is a quick snack. Much easier to pick off the injured straggler. Predators will still be cautious, because a weakened human could cause injuries. They're likely to launch small attacks, testing the prey's strength.
• Death and Disease: An injured or diseased animal may not act rationally. Rabies causes violent behaviour in many mammals. This has implications if a character is bitten during an attack. They might die from infection some months later, despite surviving the initial attack. This is one of the few times an animal will attack when it's a stupid idea and keep going despite injuries.
• Intelligence: An intelligent predator might have other reasons to attack. Perhaps someone is paying it. Insert all your usual sentient motivations here. If your predator is awesomely intelligent, don't make it stupid. It would use tactics and cunning, not just blindly blunder into the party and hope for the best (unless it's really optimistic or suicidal).

The golden rule: For healthy animals, the reward should outweigh the risk. They're not going to stick around after someone smites one of the pack with a fireball. Animals with problems (sick, injured or insane) might keep attacking despite the odds, but it's going to seem odd if this keeps happening (unless your main character is a insane animal magnet, due to an ancient curse).