Why are there pixies, etc. on the First but not the Source?

[Brightamethyst]

On the other hand the First doesn't have Sylphs, Namazu, Ananta, Kojin, Vath, Vanu Vanu and even Moogles. Some of these I'm guessing existed before the Flood but there is no way to know which ones.

The first probably had most of the same tribes as the source. Barring the Allagan created ones (ixal, ananta, possibly sylph and gnath) and the dragons (who first came to Hydaelyn post-sundering) all the others probably existed on the first at but were lost in the flood.

Maybe they even had other beast tribes that don't exist in on the Source but those were also wiped by the Flood. So Pixies are probably their own thing that only appeared on the First due to whatever made them come to be, same with Nu Mou and Amaro.

We only see what we see and we don't know what we don't know. Maybe the fae do exist on the Source and we just haven't seen them. Maybe they existed in the past but were wiped out in one of the calamities (the Vanu got their image of Bismarck from somewhere, after all.) We just don't know.