In Spore, you will meet many different species across all stages. Every single member of the species is exactly the same.*
Cell stage Species[]
In the cell stage, you'll encounter several species of cells. Strangely, unlike all of the stages afterwards, all of the species of cells you'll meet across all save games are the same few Maxis species, except, of course, for the player species. This means that none of the cells you made in other games will appear. It is believed that Maxis intentionally did this to balance out the cell stage so there wouldn't be many cells with multiple weapons. Likewise, you can never see other players' cells, even in the Sporepedia.
If a Maxis species appears with a part that your cell doesn't have, when that cell dies, you have a chance to collect that part and place it on your cell the next time you enter the cell editor. These parts are the spike, jet, cilia, proboscis, poison gland and electric bulb. For more info, see the completist achievement.
Creature stage Species[]
In the creature stage you will find creatures in a nest. All creatures of the same nest belong to the same species. You can choose to befriend the species or kill them to extinction. There will also be rogue creatures and epic creatures, which are way tougher than normal species.
Very rarely, you will find an epic creature that is the same species as a peer creature or a rogue. If you befriend the peer species or the rogue, the epic of that species will act passive and friendly towards you; however, you cannot befriend the epic through socialization.
Tribal stage Species[]
In the tribal stage you will see wild animals in significantly smaller and closer nests, which you can kill for meat or domesticate.
You will also see different tribes, which have a specific color as their name, such as pink or cyan. These tribes are also different species - possibly an allusion to neanderthals and other human relatives. You can decide to befriend them or to kill them and destroy their village.
*In the tribal stage, alpha creatures in nests have a slight color change to further differentiate them from normal creatures of that species.
Civilization stage Species[]
In the civilization stage, tribes will still be around, but all you can do with them destroy their village for a profit or to recruit a new vehicle for free, or you can wait for the tribe to become a city. Creatures will also still be around, but they only exist for aesthetic reasons, and the only thing you can do is run them over with a vehicle. You will also meet other civilizations with your species, only with the detail color matching the thematic color of the civilization. It's up to you to capture their cities religiously, with military force, or to buy their city.
Global Merger - Spore Civ Stage
This is the global merger being launched in the civilization stage. Once used, the civilization stage is complete
Space stage Species[]
In the space stage, you will meet species of all the previous stages other than the cell stage. Strangely, these species can be small and primitive compared to the higher-level species you find at the end of the creature stage or throughout the tribal stage. You will also meet different species that are part of empires. It's up to you to befriend them or kill them off.
*In Galactic Adventures, the player can make their species' captain's outfit different from the rest of their species; notably, they can equip captain parts such as weapons. For some reason, NPC empires can consist of a captain as the entire species, possibly due to an oversight.