The term creature in Spore refers to the organism controlled by the player. The term is mostly relevant to the Creature stage, because that is the stage where the player controls a single creature. The creature begins the game as a micro-organism (microbe).
In the Cell stage, the player interacts with the world with a "cell" (although it is closer in appearance to a piece of zooplankton). The objective here is to consume food to gather "DNA Points". The cell can then find a mate and lay an egg. This takes the player to the Cell Editor where one can give different abilities to the cell by adding parts, ranging from stronger weapons to various forms of movement.
As the generations go on, the cell grows larger and the camera pulls out until the microbe becomes a multi-cellular organism, most often referred to as the creature.
Each creature has its own set of genetics, which is (in practical terms) the procedural code compressed to a small size which generates the 3D creature on a player's machine. You cannot create a creature without a mouth, as it needs food to survive. At least one mouth is required to live.
Creature size Edit
Creature sizes range from mouse sized to truly gigantic. The player cannot create an Epic Creature without mods (in Space stage the player can unlock the Supersizer tool, and can make Epics in the adventure creator), the largest creature so far known in Spore, as the game uses procedural generation to create them. For example, a normal creature might be as tall as you, while an epic creature can grow up higher than the trees. The epics can be destroyed more easily in the space stage, due to the fact your spaceship can run away faster and has stronger weapons. Hyper Size creatures are bigger than Epics, and may exist on vast, unknown worlds.
Types of creatures Edit
True amphibious creatures were cut from the final game, but all creatures can swim in water around the world.
Aquatic life was originally included in the game as an option for the player's creature, but they were removed due to animation difficulties (player would need to navigate in three dimensions underwater).
A glitch, however, can sometimes cause a player's creature to generate underwater. If the creature dares venture onto land, it is eaten by the sea monster, which emerges from the dirt rather than the sea.
This does not mean that aquatic creatures do not exist on the planet- example include fish caught by tribes, the cell stage and the Sea Monster, but rather the player cannot play as one. Some might say the Sea Monster that barrows up from the earth (dirt or land) when glitch is in effect is called the "Dirt Fish" or "Monster From the Underworld" but it is currently the "Glitch Monster of Water"
Any creature can fly if given a single or pair of wings, which are available in the detail section of the creature creator, and can only be used effectively in the creature stage, however a creature will fly long distances in the tribal stage. The level determines how long one can fly, and the skill is called (more appropriately) glide.
In the GDC 2005 presentation about Spore you can see a little creature flying away from behind a rock after the Willosaur escaped from the spidery creature and Will Wright says "...on the other hand this guy is kinda fast...", talking about his Willosaur. Also, a more defined bird-creature can be seen in the presentation as the Willosaur emerges from the water and onto land, the bird is seen on top of a tree and flies across to reappear again to fly above another tree and into the distance.
According to Gamespot it was seemingly impossible to create a flying creature during the E3 playable demo of the Create a Creature system.
Flying has been included in the game, though it is posssible to fly forever through a glitch in Spore: Galactic Adventures. If the player's creature obtains a speed powerup, then flies with a glide level of 5, the creature will glide forever, provided the played holds the left mouse button down.
In a demonstration at Gadgetoff 2007, Will Wright created a winged creature in the creature editor. Despite having large wings relative to its body size which displayed some functionality once in the game world, i.e. they flapped as the creature walked, the creature did not ever take flight. 
However an article published on the 13th of February 2008 at bit-tech.net claimed that placing wings on a creature would allow it to take flight.
In the final game, true flight, like amphibious creatures has been cut. Like amphibious creatures an element of flight is still in the game with the creature creator having four types of wings that allow the players character to 'fly' for different lengths of time. Only the highest level wings allow a creature to achieve anything resembling true flight.
In the new expansion pack for Spore - Spore Galactic Adventures, the player's captain can be outfitted with a jet pack. If the hero flies too high, lightning can strike the player's creature on planets with high atmosphere.
Subterranean creatures are not a feature in the game. However, creatures can go underground in the Adventure creator, using caves etc.
Aboreal creatures are not a feature in the game. However, creatures can be made to walk on trees in the Adventure creator, but after a while, they may fall off.
Rolling or slithering Edit
Blobs and snakes exist in large numbers in the spore universe.