Even if we discarded all the previous comics and the visual book that were released before 2017, one would still be incorrect because the gen 4 designs didn´t exactly look like the ones that ended up appearing in the final product.

First of all, we have to consider that around 2015 , there would be plans for a movie unrelated to EQG, so this means that there were at least two years behind the development of such production. It ended up being distributed by Lionsgate. I am mentioning this detail, not because it is superfluous, because the MLP The Movie (2017) could have been distributed by Sony. I don´t have the emails that Meghan McCarthy sent to the Japanese company but the seapony word appeared in between the paragraphs and one of the reasons that could have prevented this offer was that the Sony employee asked what the word alicorn meant.


So, avoiding a very serious bullet, the production was already happening while they were finding another distributor (Lionsgate), and this implies that there were several concepts.
For example, Tempest was going to be called Cosmos at first. Look at that leftover name now because it would be later be used for the red draconequus that would end up in an IDW arc.
Same applies to this species. The characters that would define the design first would be both Queen Novo and Skystar. After those two, the rest of the species would follow their body type. Supposedly, the movie was going to have more action, saving Equestria because of them, but that would end up being discarded so the mane 6 would save the world instead (with more violence by the way, this decision could have changed the age ratings)

And you look at their concept art, both in the hippogriff and seapony form, and you can tell a few differences, especially in their mouths and the colors. They still had the body type defined though: the fish tails, the front hooves with fins, the small bioluminescent light and the dorsal fins (applied to the cutie marks of the mane 6) would stay the same in the final design. Just that they looked too alien and thus, the art department had to change the perspective and suit them more for this universe. Otherwise, we would have ended up with an even more exotic orthodox design that only the changedlings could reach at the time.