I LOVE designing the Hippocampus (Hippocampi? I never got that right in Latin). I can go all crazy with the designs and patterning.  
Not a big fan of the crowns the royals wear, (how are they supposed to stay on?) so I changed Novo’s crown to golden fins growing from her head. Also gave her a gradient from light purple to white, partially for aesthetic, partially for camouflage. Think of the white belly of a Great White. Also slitted pupils to make her look more predatory. I’m not quite sure if the movie mentioned anything about what Hippocampi (?) eat, but I like to see them as piscivore. Also made her tail fin a bit more sturdy, like a mix between a fish and a dolphin. Don’t know why, but I never like it when big underwater creatures are designed with the thin tailfin or fish - to me, they always look to flimsy to actually be able to propel the creature forward.