This depends on how you want to store your behaviour. If it is gonna be an AI script or just a load of booleans, for instance, they can be changed on a per-instance basis, no separate subclassing needed. But if there are gonna be fundamental differences (eg. lots of different functions) you will probably want to encapsulate the common stuff in a base class and derive subclasses for major changes. Eg:

class Orc
{
// Orc graphics stuff
}

class OrcWarrior : public Orc
{
// AI dictating battle tactics
}

class OrcScholar : public Orc
{
// AI dictating library dwelling tendencies
}

And so on. I had a similar concern in whether to use instances of the above classes for individual creatures, and thus derive them all from a base Creature class, or instead to use a separate Creature class which contains a pointer to one of the above ''type'' classes. I am pretty much decided on the pointer method, as it is more flexible (run-time metamorphosis of types! Neat...), but your mileage may vary. You may pick up some ideas from my other thread in this forum, or maybe not, depending on what you need to know