Trying to go for less of a traditional merman, I went ahead and combined features from The Creature from the Black Lagoon, Seahorses (of course), Abe Sapien from the Hellboy series and a high fantasy aspect to them. Perhaps this species has an organised military system in place.
With the outfit I had to think about what kind of materials might be readily available for them to use while under water. Shells and long bands of seaweed came to mind, as well as broken wooden poles from shipwrecks. I alsp took some inspiration from the Naga from the Warcraft series.