Terraria has a lot of monsters, some are in almost every game (skeletons, zombies, slimes), but they seem to be missing on all the extremely obvious choices which would fit in perfectly! Dragons would work well in the Hallow, giant spiders' absence seems strange, and other such things, but there was one thing I thought they should really have: Merfolk. And I don't mean the friendly fun and girly mermaids and mermen, I mean the ones from myths long ago that lured people into the water and then ate them.
The merfolk would look like how you'd expect them too, and some mermen weild tridents (though they don't use them). They spawn in the ocean and have an uncanny habit of spawning in groups. When in the water, all the merfolk on screen start to home in on you, and one comes from offscreen for each one on screen (if three were onscreen at the time you got in, another three would come off screen and attack you, too). If you are out of the water for more than 20 seconds afterwards, then the next time you get in the water, they will multiply again.
The mermen have better defense than the mermaids, but the mermaids have better attack and slightly batter health. Though both drop merscales, only mermen drop tridents, and only very rarely.
After killing enough merfolk, you may get the Fishy NPC! The fishy NPC can only survive in water (so his house will need it) and he sells multiple sea-based items, such as the breathing reed, flippers, and though expensive, the Neptune's Shell.
That was my idea! Feel free to use it in any of your mods if you give some credit to me, and post suggestions and comments BELOW!