Now, there were plans to introduce them in gen 4, back in the golden years even.

However, ask yourself this question? Do you think that they would have received the gen 4 designs that appeared in 2017?

Technically, when one is talking about the gen 4 designs, that person is correct and anyone would understand that said person would be referring to the designs that made their debut in the 2017 movie. Just that... it´s not 100% correct. Gen 4 displayed several attempts that would come in several formats.

Namely two formats:

-Under the Sparkling Sea in 2013, a visual book whose story consists in the mane 6 visiting Aquastria and their designs would resemble more the body that appeared in gen 1
- IDW comics.Issues #3 and #13-14. For issue #3, you would get the hippocampus design (that is to say, they would add here the limbs: fish tails and front hooves) and for #13-#14, they would revive the gen 1 design with the artsyle of gen 4. As the cover suggest, they would translate quite well into the gen 4 universe.

However, this species is the closest thing as requesting an artist to draw anthro ponies: how do you draw their bodies? What are the characteristics that everyone should draw and thus, reach a consensus on how their design should be the standard for everybody?

Because of this, the development and attempts to revive them have turned out to be inconsistent and there were failed attempts. There was a small amount of interest in order to bring them back again but they weren´t a priority. In fact, because of this, the word merpony (mer is sea in French) would exist and thus, you would distinguish the seahorse design of gen 1 from the hippocampus design that could have been portrayed in gen 4. However, there is also a tricky question over here, unlike the seahorse design, you wouldn´t have any reference in order to draw merponies and you will see fan content in which they appear with hind hooves and the design of their tails could be quite diverse.

So, while there were attempts and plans to bring them back, they weren´t a priority nor Hasbro would see them interesting enough in order to give them another try.