Each world chooses a certain number of werecurses to potentially exist, usually determined by world size. How many curses actually exist in the final product is determined by historical events, and is usually significantly lower.
I thought they'd just be an animal. From making Primal, I generally found that most animal predators follow the same general rule. Outside the extremes of weakness and strength, a concentrated attack will almost always kill civilians, but lose to actual soldiers. Generally, only overwhelming numbers or favorable circumstances for one side or another break that pattern. It's Dwarf Fortress though, so there's plenty of flukes in real combat.
I think the reason for this is that animals can't gain combat skills. As a result, even Novice Dodge and Block skills represent a major advantage against them. They also can't wear armor, so they just take full hits from whatever weapon the soldiers use. On the other side, civilians usually don't have much to throw at animals, and they often try to run. That gets them killed, since they usually can't put up much of a fight and the animals are faster than them anyway so they just waste actions, get tired out quickly, and then the animals bring them down. How strong the animal is precisely typically just makes a difference in how long the kill takes.
There is a major exception in that animals often don't put much effort into their attacks. Many animal attacks come from a predator simply approaching a dwarf, exchanging a few blows, and separating. In that case, it does make a major difference if the animal is capable of killing or maiming people in a few swings, and if the animal has only edged attacks. If it doesn't have blunt attacks, they tend to escalate past the lower-level combat stages to lethal combat much quicker.