It sure is hard to Na'Vigate the world of fan theories sometimes, but this particular one is worth taking a look at.
Reddit-user ParameciaAntic has posted this Avatar fan theory, implying that the Na'Vi are actually not native to Pandora, but they somehow colonized the planet:
All of the other large animals shown on Pandora display a hexapodal body plan, more than two eyes, multiple breathing apertures along their necks, four digits, and have two nerve stalks.
The Na'vi only have four limbs, two eyes, apparently only breathe through their mouth and nose, three digits, and only have one ponytail nerve stalk. They appear to have non-native origins, possibly genetically engineered to interface with the local flora and fauna.
Maybe they are actually descended from avatars of another alien race that visited Pandora long ago? [...]
I think the Na'vi were engineered by an alien species to adapt to life on Pandora. They may be the aliens themselves genetically modified for life in this environment, with the added feature of the nerve connector. Alternately, they may be a wholly artificial creation, analogous to the human avatar program, patterned on the DNA of the native animals.
I have to admit, it does make some sense! The Na'Vi seem very different from all the other animals we've seen on Pandora. The closest thing to a primate is the monkey-like Prolemuris, which is described as an evolutionary missing link between the Na'Vi and the other animals:
I do find it a bit odd that the Prolemuris and the Na'Vi are the only primates on Pandora, but then again we probably haven't seen all the flora and fauna yet. I wouldn't be surprised if we got to see more diversity in Pandora's animal kingdom in the sequels, and if we're lucky we might even see a space chimpanzee.
Humans don't look like platypodes
Let's not forget that a popular belief amongst scientists is that all animals stem from the same ancestor, which would mean humans are a distant relative of, for instance, the platypus. You and I don't look anything like a platypus, but that doesn't mean we don't belong on this planet! (Unless you're a shapeshifting lizard alien of course, in which case the human race would kindly request you to leave this planet.) Therefor it's safe to assume that the Na'Vi might still belong on Pandora, even if they look different to many of the other animals. I think this is a fascinating fan theory, but it really depends on what primates we will see in Avatar 2, whenever that will be released.
If this fan theory has got you craving some Avatar goodness then you have come to the right place. Here's a clip from the movie showing Jake Sully being chased by a vicious thanator!