Here it comes the biggest plot twist that could have changed their ideas.

While Rainbow Rocks didn´t appear beyond a few theaters in the US, one cannot deny that the sequel of Equestria Girls would get a much better reception than the original. Mostly because of the redemption of Sunset Shimmer, which would forge the path of reformation of other characters later on Friendship is Magic.

However, there were apparently three new human characters that would come as the villains: the Dazzlings.
What made Rainbow Rocks more successful among the fanbase were the songs that Daniel Ingram composed, the redemption of Sunset and these villains right here. Both Adagio, Sonata and Aria would shine by singing songs that would corrupt other people and thus, cause disorder, chaos and conflicts. Beautifully sung and appealing but genuinely a cold headed move from their part.

Turns out that in the end, they would reveal their actual bodies and thus, you can tell that their amulets hold Equestrian magic, mixing even more the dynamic that fans were used to seeing in Friendship is Magic.

If one reads the wiki, Hasbro didn´t have plans for seaponies despite acknowledging the mermares from the 2013 visual book. However, given that EQG would soar with this 2nd movie and gain a lot of popularity within the community, testing the waters that EQG could offer even more than a one time special, one of the reasons why it shone were the sirens. In fact, you look at their songs and the number of reproductions of Under Our Spell among other reach millions, becoming even as popular (or even more) than This Day Aria.

This would pay off with Shadow Play in season 7 (episodes 25-26) and the Legends of Magic arc, where you can see that Starswirl banished them from the magical land, justifying in hindsight their existence.

But the designs were there. They would look close enough to the hippocampus design and perhaps, this could be the turning point for considering a production with the aquatic species.