The following is an analysis on how creatures behave in the story of Duel Masters.
Creature lifespan tends to vary between individuals, some such as Living Dead from the ancient Darkness Civilization tend to be very short in lifespan, spanning their life in less than a few months to years only to be resurrected again, while others such as Aquan, Gyuujinmaru, Legendary Identity and Prin had been living on for tens of thousands of years with their appearance barely changing. Others such as Dragonoids and Humans share the same lifespan as regular earth Humans.
Most creatures do have a society structure, most notably the Civilizations shared between the creatures themselves. Early creatures tend to live in military advanced societies such as in the case of the Ancient Light Civilization, and the Ancient Fire Civilization, or primitive tribal societies such as Nature Civilization.
Later on creatures that base their society on advanced Monarchy existed, in the case of the Aliens and the Dragon Saga Dragons. Others such as Oracles and Outrages, run earth human-like advanced societies that are based on a various value. Various races, such as Human Baku and the aforementioned Oracles are known to have human world-like behaviour such as Education and Marriage.
The average creature is capable of defeating a regular human in combat and are the equivalent of the most powerful earth human soldiers.
However, many do have the ability to control elements, manipulate minds, cause disasters, create advanced weaponry and even destroying entire planets by themselves. Sone even have powers that are equal or exceeding those of Gods.
Note that a creature's strength level is not nessecarily its card power; Low card power cards such as Supernova Black Hole Thanatos and Godfather, D2G are known to be very powerful in the background story.
Interaction with Earth Humans
Creatures tend to be cautious and hostile against Humans unless they are used as cards.
However, some are capable of befriending them, most notably the Kirifuda Family in the Anime. Others such as Jokers and Revolutionary creature spirits, are shown to be friendly around humans and would happily cooperate with them.
In the other hand creatures such as Devil Diabolos Zeta, Evil Awakened, Izumo and Dokindam X, The Legendary Forbidden seems to have no other reason to interact with humans other than to destroy them or achieve a benefit that will destroy the balance between the creature world and the human world.
However, in Joe Kirifuda's era, the interaction between the human world and the creature world has increased as Specials impose themselves as humans for unknown reasons, Jokers are created into this world and Boltz being assisted by members of the Fire Civilization such as Dachicco Churis and Bad Brand.
Creatures have different morals than those of Humans. Each creature, especially each linked society of creatures have their own values. Most see reckless destruction in their own way of enemies as a key to survival, while others like Aquan, Sapphire Wisdom etc. use intelligence and schemes to resolve conflicts.
This makes them extremely troublesome when imported to the human world. For example, in one case a Kiradon who entered the human world wanted to duel with humans but ended up petrifing a gang who steals rare cards.
While unknown if Deckie is a creature, he also came from the creature world and had very different values from humans unless when Joe teaches him human values.