[TOTK] [SPOILERS] are Rauru and Mineru truly the last of the Zonai?

I think it's important to remember that the Ancient Hero's Aspect is just that, an aspect of the hero that Link equips. It may be no more accurate to the "real" Ancient Hero than Zora Link was to Mikau, for example. If Nintendo ever decides to depict the Ancient Hero in the flesh, they could look very different - I personally suspect they'd be taller, with different proportions, and actually blinking and emoting instead of having an uncanny static face. (Seriously, that still freaks me out...)

That said, the Aspect itself does look closer to a Zonai than any other race we've seen so far, with the dark metallic skintone, long snout, large ears, armor and bodypaint. The only other race they could reasonably be is the unnamed Depths race (though their statues don't have tails); since that race interacted with the Zonai, it would still make some sense for the hero to have Zonai armor. The lack of horns, digitigrade legs, and long tail are outliers for a Zonai, but given that we've only seen two of the species, it's possible that their appearances varied. And Rauru and Mineru may even have had vestigial tails, for all we know.

So, if the Ancient Hero was a Zonai, how could they suddenly appear during the Calamity after Rauru and Mineru were gone? There are many possibilities: time travel, some kind of stasis (like BotW Link who slept for 100 years) that led to them being forgotten and then rediscovered, or possibly there was a lost colony of Zonai that nobody knew of, living out in the wilderness. I dunno, Zelda lore has had wilder moments. But we'll probably never know if they were actually a Zonai and where they came from, unless we ever get a game about the Ancient Calamity.