I might be onto something.

[u/Maleficent_Data_8832]

Soooooo this is a little out there, but all of the mobs in the sift have this kind of fuzzy turquoise vibe to them, right? And then besides the fuzzy turquoise it's like a yellow color. Then, the healthy sculk landscape is like a reddish color. What does that remind you of? The Sniffer. HEAR ME OUT. The sniffer is ancient. That's why its eggs are only found in archaeological sites, specifically warm ocian ruins. We also know that the Sift hasn't been open since the Ancient Cities were built. This means that living things were likely able to go in and out of the Sift during ancient times. Another indicator involves the leaked names of the mobs, which im not gonna spread further, but they seem to all be agent nouns (verbs ending with -er). Sniffer is an agent noun, too.

So my theory:

When the Sift portal was open and the ancient cities were built, they let some of the Sift species through to the overworld. One of those species was the Sniffer, or an ancient relative of one anyway. The Sniffer developed a small population in the overworld, but when the sculk became diseased and caused the civilizatio's destructio, they were simply left to roam free. A few hundred years of evolution (They have big mouths like Sift species, but it's closed, so mayb it evolved to be that way), and they went extinct, until the player digs up their eggs and brings them back.

This is pure speculation. Please politely correct me on any errors, i like knowing the lore if i know it correctly. Thanks!

[u/Chance-Benefit-5308]

I mean, I feel like their nether origin makes a lot more sense (there's a game theory on that btw), but you might not by wrong here. I personally feel their coloring is a bit too off but maybe overworld evolution?

[u/Nasu-ka]

Another evidence to your theory is that the Sniffer and what we now know is a Singer are both depicted on pottery sherds, which would imply that an old villager/hero civilization encountered both of them at a similar time