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

[Lumivyory]

While there are some beast tribes that exist only on the First, there are also tribes that exist only on the Source.

Other than the Pixies, the Source also doesn't have Nu Mou, nor do they have Amaro (which technically are their own tribe with those that regained intelligence).
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. 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.

[Enkidoh]

On the other hand the First doesn't have Sylphs, Namazu, Ananta, Kojin, Vath, Vanu Vanu and even Moogles.

Moogles do exist on the First as well - they have blue poms and are seen even running a version of the Moogle Delivery Service (presumedly G'raha established it there based on the Source's one, but it still leads to questions as to how that can connect with the Moogle Delivery Service on the Source - presumedly it's similar to how retainer services are handled by Feo Ul, but that's a long shot).

Ananta and their lamia cousins not existing on the First makes sense though, given those were synthetic creations of the Allagan Empire same with the giants/gigas and so never had a counterpart form on the First when the Sundering happened (although it's possible some might have hitched a ride to the First via the Crystal Tower like the gremlins did). But as you said, a lot of beast races on the Source not being seen to have counterparts on the First can be just chalked up to being rendered extinct by the Flood of Light.

[Lumivyory]

The first had namazu like creatures in ronka or came to ronka via the Qatari or rat race before the flood. You learn that in their beast tribe quests

Moogles do exist on the First as well - they have blue poms and are seen even running a version of the Moogle Delivery Service (presumedly G'raha established it there based on the Source's one, but it still leads to questions as to how that can connect with the Moogle Delivery Service on the Source - presumedly it's similar to how retainer services are handled by Feo Ul, but that's a long shot).

Ah, totally forgot about these lol

And I suppose we could still encounter Source's versions of Pixies or Nu Mou when we venture into the new expansion, or it could be completely new beast tribes. For now we can only assume that they either don't exist or aren't in any area we have explored.
To be honest I'd like them to stay exclusive to the First, it's nice to have some differences.

[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.

[Kuroka]

While there are some beast tribes that exist only on the First, there are also tribes that exist only on the Source.

Other than the Pixies, the Source also doesn't have Nu Mou, nor do they have Amaro (which technically are their own tribe with those that regained intelligence).
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. 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.

Amaro are made by the Ronka mages, before they basically were just animals. No mou could be still out thereor just died out, we cant tell yet...

Sylphs might be the same then pixies, we know they dont give birth like we others do, so it might very well possible they are just the same as pixies, just evolved different due our calamitys and less light aether.

As said by others there are Moogle and those who did the Qitari beast tribe learned they encountered namazu - dont forget their native region just as the Ananta was amont the places destroyed by the light...

The Ananta are a native tribe of the saurce as well, its mentioned they were the base of the allagan Lamia...
Though we do know the Ixal are artificial made... as well might be Vanu and gnah, but they might have just evolved randomly as well.

Given that the Fae of the first can go between worlds trivially, they could pick to be anywhere, but choose the first. Maybe they actually thrive on the high light aether concentration?

They can only after making a contract with us, someone whos aether they can trail there. As she explained they basically become a extension of us, thus allowing that. Normal creatures cant do that as we cant just take others between the worlds (at least not wit massive magics and tech).

[YianKutku]

EDIT: Oops, I didn't see the new page, so my original post (that there are Moogles on the First) is redundant.

I admit to some curiosity that the Viera on the Source are mostly in Othard, while the Viis of the First are in Norvrandt, half the world away. Given they have lengthy lives and are also deeply insular (and thus not likely to migrate), I have to wonder if this is actually a decision made out-of-universe to allow for the newly-designed race to appear in the expansion, rather than actual lore.

[Brightamethyst]

I have to wonder if this is actually a decision made out-of-universe to allow for the newly-designed race to appear in the expansion, rather than actual lore.

It's almost certainly primarily a gameplay decision, but there are any number of legitimate lore reasons a move could have happened if they ever need to explain it. For all we know the Viera are native to Eorzea but moved out in the wake of one of the calamities.

[zlygork]

EDIT: Oops, I didn't see the new page, so my original post (that there are Moogles on the First) is redundant.

I admit to some curiosity that the Viera on the Source are mostly in Othard, while the Viis of the First are in Norvrandt, half the world away. Given they have lengthy lives and are also deeply insular (and thus not likely to migrate), I have to wonder if this is actually a decision made out-of-universe to allow for the newly-designed race to appear in the expansion, rather than actual lore.

Obviously, the meta reason is that they want exposition to the new races in the new content, but I suppose the non-meta difference would come down to the Ronkan empire whose knowledge they safeguard. Either it doesn't exists in the source, or all of its ruins have been long lost to one of the many calamities.