Cunning is a guide to help you judge the intelligence of your creature. It does not effect actual game-play, but I felt it was an important stat to have (in comparison to mentality, which is for magic, psychic abilities, and willpower).
Generally does not know how to defend itself, or react properly. Is usually too trusting, or has no concept of trust at all. If it rains, the creature will likely stand in it, rather than move to get dry. Can learn to follow commands, but is difficult to teach, and follows out commands like a robot. If you teach it to "go", it may not ever stop until it dies unless you thought to teach it to "stop". It cannot make that judgment call on it's own.
Critters know enough to move out of harms way, but generally have no fore-sight. They are not like rats in a maze, and would likely starve if put though such a simple test. Beetles are generally creatures associated with the "critter" level of intelligence. Teaching a critter to follow commands is usually done by bribery with food.
An animal is a creature of lesser intelligence than a human, but is clever enough to think up it's own tricks. After all, a dog may learn how to open a door, or jump onto the counter to get the goodies when you're not looking. Most animals fit into this category of intelligence and they can be taught many tricks with a good trainer. They usually understand the company of other creatures, such as humans, and generally enjoy it.
A beast is a creature that is considered non-sentient yet borders on an intelligence that rivals humans. Many people think gorillas are unintelligent, yet they can learn to use tools. Parrots too, are considered unintelligent, yet they can learn how to communicate with us (while we cannot learn to communicate with them). Beasts are very intelligent - more so than a common animal, yet they still do not have their own society. A human my learn to build a civilization, but a beast will continue to be wild on it's own accord. Such creatures, as clever as they are, are considered beasts.
Sentient creatures are those that have a keen enough intelligence to comprehend language, even if they cannot speak it. They have the ability to understand tools, even if they have no means to use them. They can think up of their own ways out of difficult situations and traps, and can usually use complex strategies against their prey or foes. They are usually a people of a society, if not a creature that is of human-like intelligence. A creature of Sentience (or greater) is usually considered a slave when owned by another.
A mystic is always a creature of greater cunning than just any other Sentient being. Though they are sentient themselves, they have a much wider understanding of the world - a connection there-of. They are usually the magic-users of the world (though many sentient creatures can learn to use magic), as magic seems to almost come naturally to them. They are the great puzzle-builders and magic-weavers, or even those creatures that consider themselves pious and can simply understand everything's place in a much cleaner manner.
Divine creatures seem to have the mysteries of the world wrapped around their little finger - assuming they have one. They have an understanding of the world that humans cannot hope to possess. Not many creatures can understand the way of the Divine intelligence, and creatures of Divine intelilgence usually look upon less cunning beasts with pity or apathy.