I think there is something else at work

There’s currently three more of these.

@Background Pony #0CFC  
Totally agreed about that, and I have been following this since 2014. The question is: fish or mammal?
 
Obviously for Earth types, mammal, as they have lungs rather than gills, and can breathe just fine out of the water.
 
But the others, not so cut and dry, forgive the pun. The presence of gills, and need to keep them wet to breathe in addition to features from specific fish specie makes calling them fish tempting. But the presence of cleavage and navels suggest mammal. Unicorn and Pegasus types seem to tread a line. Calling something a fish that has mammary glands and belly buttons seems strange.
 
It would seem to me AJ and Pinkie can resume their lives business as usual this way, and the only thing to be concerned about is contagion. If that were removed, they could live normally this way, and only require slight adjustments in very specific things. The others are stuck.

@Background Pony #BBC3  

 
Basically, the thing to remember is in this story there are three types of mermaid, each one correlating to the tribe of their pony selves. Fluttershy & Rainbow Dash are the quote-unquote “pegasus” type of mermaid, with gills behind their finned ears, and possess webbed fingers as well. You could say they are the most fish-like of the three types, what with the high amount of finned/webbed tissue, as well as the fact of the three they can spend the least amount of time out of water, drying out relatively fast. Conversely though they are the type most adept at existence in the water, able to swim at lightning fast speeds and leap from the water to great heights.
 
Then you have the “unicorn” type mermaids, of which Rarity & Twilight are (Twilight is considered a unicorn type despite being an alicorn now because she had started her life as a unicorn). As you can tell, they have their gills on their necks and that angler horn. So far, they haven’t figured out the function of their horns yet, though it has been indicated it doesn’t work like a unicorn horn, nor is it simply a typical angler horn. Compared to the “pegasus” type, “unicorn” type mermaids can spend much longer out of the water, but still can’t wholly divorce themselves from water entirely, needing to keep their gills moistened regularly out of water. Think of them thus as the bridge between “pegasus” mermaids and “earth” mermaids, jack of all worlds, master of none.
 
Finally, Applejack & Pinkie are “earth” type mermaids, which you largely nailed in your assessment. They possess no gills whatsoever, instead having larger, more efficient lungs that allows them to hold their breath for extended periods underwater like various marine mammals like seals and dolphins, but as such this also means they must surface for air regularly or very well asphyxiate and even drown while in the water. They also seem to have a membrane in their trachea that allows for them to still speak while underwater without having swallowing water into their lungs in the process. They also are the slowest swimmers, swimming much more sluggishly than their counterparts. Instead, their strength, as shown so far, is that they operate wholly independent of water, as well as having adequate tail strength to be able to “stand” on them on land, potentially even allowing them to theoretically “hop” to and fro, as so long as the distance isn’t that far. In essence, outside of the fact they have a fish tail instead of legs (and could potentially spread the mermaid curse by touch still), they can actually continue to live and function like they were still human while on land, giving them unparalleled freedom of mobility. “Earth” mermaids in a way are better adapted to being on land than being in the water.
 
That should clarify the differences between the three types of mermaid in this series (and why Applejack is best mermaid :3).

@Background Pony #BBC3  
Well not shure, PhysicRodrigo mix many mermaid’s types here.

I love this entire series, but it would seem this will be the last one in a while, given the decreased rate these are being drawn. It may be a month or two before the next shows itself.
 
I often wondered about these. Is it a fish or a mammal? I’m more inclined to say mammal given the obvious mammal features, yet some are more fishlike than others. These two and Flutter shy all have those gills and require being moist at all times, unlike the other two. Otherwise, they’d asphyxiate. This is to say nothing of their scaly tails, though that is aesthetic. And there is the finlike ears and angler horns they all and Twi have, which tie them to a specific specie of fish. Yet they have the naval and cleavage to suggest mammals.
 
Applejack and Pinkie Pie by contrast are more cut-ad-dry mammals. Forget the texture of their tails, they have been compared to sea lions more than once, and are clearly described as not needing immersion nor having gills to breathe. AJ even spent substantial amounts of time out of the water and almost surely got dry as can get without problems, as I suspect Pinkie could too. She’d have no problem leaving the water without having to keep moist. The next step up from a sea lion. They only have tails where their legs were, and expanded lung capacity, but still lungs.