ah, Anglerfish. But I'll make it less creepy, I think I'll just adraw a pretty fish with that thing on their head. Horns are used to fight in the animal world so this should do the trick underwater ALso as for designing, flying fish is one thing but they can't do much underwater, in the underworld world they should do something that's useful like.. Speed. So more aerodynamic body and stuff. I think...? Flying fish is the obvious first choice but if we think it through that won't do much
LOL Anglerfish. Those things are terrifying-looking in real life, what with the ridiculously gigantic mouth and long, skinny teeth and all. You could probably just do a regular fish body with a sort of hybrid between the anglerfish lure and a unicorn horn, something like a unicorn horn with an anglerfish light-up bulb on the end. That oughta look cute and not creepy.
Streamlined, fast-swimming fish, huh... Try sailfish. Not only are they widely regarded as the fastest fish in the world, but they also have this cool stripe pattern down the side and this huge dorsal fin that looks almost like a wing when fully unfolded. It's practically a fish version of Dashie already.