"Earth filly" is only being used on one image right now and probably won't be used much, if ever, again. Its use is incredibly specific, and while it would be widely applicable (ie. most images of Apple Bloom), that doesn't mean it would be a net positive to have. I'm willing to keep "crystal filly" because it already exists and has implications, but I'd rather leave it at that rather than add uselessly specific tags for every combo of pony races and age groups. In addition, sea ponies are pretty ambiguously pony, since you could argue that they aren't ponies in the same way that IRL seahorses aren't horses.