>>48460063I suspect that it's actually because a Dolphin Pokemon would be difficult to get right--not in terms of design but in terms of where that design fits within a gen.
Dolphins are adored worldwide just about so a Dolphin Pokemon would need to capitalize on that and look really cool and/or be really special in some way. When making marketable, "iconic" Pokemon like this Game Freak seems to have a few categories in terms of where they go:
Starters
Pikachu Clones
Furry-bait/Waifu-mons
Legendaries & Mythicals
The problem is that when you think about it, a Dolphin Pokemon actually has a hard time fitting well into any of these spaces. Remember, Pokemon are designed to sell merch and be used in-game based on how their marketing team or whoever predicts which ones will be popular. There's a reason that Greninja is more competitively viable than the other Kalos starters
