If you couldn’t tell by the tags, he’s actually a Seapony, for multiple reasons, all of those reasons being his last name of Fish, like, c’mon. Also his signature is a fish, and it’s what I used as his CM. Also, his CM serves no purpose and has no meaning as he is not normally a ‘land’ pony, it’s just there, it exists so he’s not a blank flank.
He has no qualms being a Seapony like how he was born, it’s just hard to do music stuff underwater so ventures onto land as an earth pony thanks to the help of a necklace. However he needs to stay hydrated as an earth pony and he mainly accomplishes that by carrying a water bottle with him at all times (of which I stuck a G1 reference onto) and sleeping in bathtubs and showers at night. It’s a bit unconventional, but everyone knows what he is and how he manages so it’s not that weird. (Except to Oliver who not only is dating a Siren but recruits a Seapony into his band. That’s two too many fish.)
His necklace is real and it is his means of shapeshifting. Unlike my Siren headcanons tho, if he takes off his necklace as an earth pony, nothing happens, he stays an earth pony, tho he cannot change back without the necklace. It is also a magical necklace, yes, but it is not one of a kind and can be destroyed, which is probably a faster way of killing him if you just keep him away from water. In this universe, the logo on his necklace is from wherever he came from as a seapony, and still goes to as again, no qualms with what he is and how he was born, he has family underwater, but he also has friends on land so he balances both.
And to specify in case you cannot tell, he is a SEAPONY, he was born a SEAPONY, no Hippogriff in sight. Also as a ‘land’ pony he retains the colored pupils of a Seapony and he has a different eye and ear shape to be a little more unique as well.
I wrote up some seapony headcanon stuff a few months ago that ties into this.