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

[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.

[Kesey]

The first has moogles too. You see this in the quest series that unlocks the huntboard for Shadowbringers in the Crystarium.

[Lusavari]

I'm surprised no one is floating an suggestions of the Faeries of Nym having a relation to the Pixies. The Fairies are bound to the Scholar Job Stones, with ours canonically being named Lily. I've always been curious why Fairies don't spawn naturally without Scholar Job Stones, and it's likely many were bound. They've never got THAT deep into the fairies other than Lily previously belonging to the Tonberry that functions as our SCH mentor. And one of the other Tonberries trying to reclaim her Fairy which almost killed her due to problems with being a Tonberry. While I absolutely do see a sylphic connection, the sylphs are a sapient floral creature which could have been wiped out by the Ronkans for all we know. Pixies manifest out of thin air and are born, usually through an infusion of a child's soul. Fairies (and please someone tell me if I'm mistaken) we've never gotten a good origin on how they manifested, but they might manifest from more than just the soul's of children considering they're not as callous and lean toward wanting to help people.

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.

Someone hasn't done the Qitari story line~ I hate to throw Spoilers out there, but it is explained that at some point before the Flood, the Qitari encountered the Namazu counterparts of the first who tried to move into the Greatwood. How that interaction played out is lost to history... and for you to figure out with the Qitari. Anything that wasn't created by the Allagans may well have a counterpart. The Vath and the Vanu Vanu were probably lost in the Flood.

[Rosenstrauch]

Heck, if fairies and pixies being related opens the way for a Feo Ul/Tyr Beq fairy glamour, I'm all for that idea.

[ICountFrom0]

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?

[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.

[Lusavari]

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 don't physically manifest anywhere other than the First. Feo Ul does what she does through Dreams. Your pact allows her and her alone to bridge the gap and experience the Source through the Dreams of people close to you.

[Veloran]

Attendant to their lore as manifesting from the souls of dead children (Along with the Fuarth being souls of the drowned), it could be the pull of the Lifestream on the First is significantly less strong than on the Source.

They don't physically manifest anywhere other than the First. Feo Ul does what she does through Dreams. Your pact allows her and her alone to bridge the gap and experience the Source through the Dreams of people close to you.

Tataru says that Feo Ul appeared to tell her about you while she was in the middle of working.

[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).

[DPZ2]

Hmmmm. We didn't know about the Ananta, nor the Namazu, until Stormblood.

It's almost like there were undiscovered beast tribes that we suddenly found out about. I wonder if that might mean we don't have any idea what the rest of the world holds for us.

We know that there is at least one entire continent on the Source that hasn't been referenced by anything other than what our Blue Mage trainer has told us. Perhaps the pixies are from there. They certainly weren't in Voeburt until after the Flood.

Again, an event that occurred 10,000 years in the past, creating disparate non-synchronous realms, does not mean there absolutely has to be one-to-one correspondences.