Hope you don't mind me answering this over here rather than my fandom blog! Wanted to draw a mer!Thorne design in response ^^
To answer your question- my first thought was immediately that Thorne would be a shark mermaid! (and more specifically, a Great White Shark). Making her one of the ocean's notable hunters seemed fitting.
There's not a whole lot of species specific notes for the design itself to be made- her tail moves side to side like most non-mammal aquatic creatures, and is countershaded (not seen here but the dark coloring stretches across her back and starts fading just as it gets to her neck). I don't know if the gills should logically be on her neck instead, but I really, really love torso/chest gills. And ofc she got the teeth and a big ol dorsal fin on her back.
She still gets into a lot of fights and trouble as a mermaid, so her tail is covered in additional scars. How many of these are from humans/fellow mermaids and how many are from sea creatures? idk lol. I just know the sharp teeth definitely make her more of a menace.
But ye that's about it. Thanks a whole lot for the ask @rainbowprisimatic!! <3 It was super fun getting to draw Thorne as a mermaid lol.