Originally Posted by
LineageRazor
Died in the flood is my guess, as well, and they're not the only ones. Namazu, Kojin, Lupin - assuming they were all located in geographically similar places on the First as they were the Source, are likely extinct, or with only a bare few survivors. Seeq, and Bangaa may be gone, as well. Lamia, Vanu Vanu, and Gnath are close enough to the region that there may be more of them, but they haven't made an appearance yet.
Do we have evidence of this happening beyond Arenvald? I can't remember if there's actual dialog in-game that implies that his case is the norm, or just one of many possibilities. Perhaps Arenvald just happens to genetically favor his mother rather than his father, and that other cross-breeds could just as easily favor the Garlean side (maintaining the third eye and such).