There are several odd races inhabiting Known Space, and at least five of them are humans.

In Known Space mankind has finally begun to colonize other worlds. And in Niven's books this has had odd effects on mankind. Each colony world is unique, with different gravity, environment and culture, all of which have worked to alter man.

Flatlanders
These are garden variety Earth natives. The name Flatlander is a derogatory comment used by Belters to mean anyone who doesn't live in space and seems unable to move with the times and see the new frontier. Earthers live in a vastly overcrowded world, and as a result have slightly different customs than the other colonies. This is the main thing that sets them a part as Flatlanders are much the same as they are today.

Several aspects of society have changed. Due to the massive overcrowding humans must apply for permission to have children, the laws designed in such a way to prevent anyone with less than perfect genes from having offspring. This fact coupled with the sheer numbers of people have loosened the moral strings of older times. Relationships are often fluid and open, with legal contracts being made for more lasting and meaningful relationships, often based around the bearing and raising of children. Flatlanders are often considered promiscuous by other groups, specifically the more uptight Lunies.

As with all peoples there are flatlanders who work in space, but there is a fairly unique phenomenon on earth. Called "Flatphobes" they are humans who are pathologically afraid of leaving earth, unable to even considered living on a different world with different air, gravity and life.

Lunies
The Lunies are native to Earth's Moon, Luna. They have adapted oddly to the reduced gravity of the moon. A typical Lunie is easily 8 or 9 feet tall, with extremely long limbs and fragile bones. They are experts at moving in low gravity and seem very much natural in their dome cities. However they are pretty much unable to normal visit any of the other colony worlds, their fragile skeletons unable to support their weight.

Luna is only partially independent of Earth, to reliant on supplies and support to truly break away. The population to small and too new to be completely independent.

Lunie culture is very staid and formal compared to earth. It is considered very bad taste to make a pass at a lunie that isn't guaranteed to be successful, or in otherwords that isn't invited first. The lunies consider nudism to be offensive in public and the impression of promiscuity is a social taboo.

Belters
Belters are normal humans who live and work in the asteroid belts beyond Mars. They look the same as flatlanders due to extensive use of centrifugal gravity in their habitats. The main difference is one of culture. Almost all Belters wear functional hairstyles and little in the way of clothing, nudity is practically the norm. All artifice for a Belter is confined to the outside of his spacesuit, which might as well be his true home. The Belters are experts in low gravity and starships. They consider themselves to be very much independent of Earth. Finally a true Belter will almost always take a metal prosthetic over a organbank replacement.

Jinxians
Jinx is a moon in orbit around the planet known to Jinxians as "Primary", an orange gas giant. Jinx itself has the highest gravity of any of the human colony worlds. As a result the natives are short, stocky and very powerful. Well adapted to an environment where even normal humans are unable to move effectively.

Crashlanders
Living underground on the originally named "Crashland" is another colony of humans. Crashland has low gravity and ferocious winds. The results of which all natives live in underground cities, and only go to the surface during the quiet seasons. As a result they are almost always albinos requiring Melatonin tablets to survive on most other worlds. They are also attenuated like the Lunies but not to the same degree. A Crashlander might be as much as 7 feet tall, but is strong enough to walk under his own power on Earth.

Other Colony Worlds
There are at least 3 other colony worlds but I know very little of them. They are Shaster, Home and Wunderland.

They always keep their contracts, but blackmail is a way of life to them. A race derived entirely on the guiding principal of cowardice. Technologically advanced beyond the belief of man, they will sell anything but the cost is usually so far beyond what anyone..even governments can afford. The Puppeteers through the corp known as "General Products" supply the races of Known Space with the best starship hulls in existence (only some 10 trillion stars for the secret). They are willing to pay any price for silence to anyone who learns anything about their homeworld.

A Puppeteer is an odd creature to look at. Three legged it walks with a weird hopping lurch, two guide legs catching it when the main power leg pushes it forward (and despite the cowardly nature its power leg makes a very effective weapon). Its heads are also unique, frilly appendages on its lips doubling as the creature's hands, at the end of very flexible necks. Both heads are equipped with sensory organs and the ability to speak. And what voices. The Puppeteers speak all the languages of their client races with fluency and oddly beautiful voices for such odd creatures.

At present the Puppeteers have left Known Space, taking their worlds with them. Seemingly fleeing a cosmic explosion which will consume the Milky Way Galaxy within the next 20,000years.

Ah the Kzin...one time enemies of man, and perhaps future enemies of man. Feline humanoid, like 6 foot tall Tony the tigers. Ferocious and strong, but foolhardy and reckless. They advanced across space solidly till they encountered the humans and learned they weren't the strongest after all. Four wars where fought in all, and each time they moved to soon and lost.

The oddest of the races, sentient plants that exist entirely in space. Enigmatic and reliable. The Outsiders are known for their proverbial nature in living up to signed contracts. They look much like black daddy long legs, a small body equipped with expelled gas thrusters and long thing arms to manipulate things with in their entirely zero-G environments. They possess the fastest realspace drives in existence and are seemingly capable of moving nearly any mass, its been suggested they are responsible for accelerating the Puppeteers worlds to near light speed to escape the destruction of the Milky Way.