I hope this idea hasn't been done to death. Sorry if it has.
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Name: Angler Seaweedle (latin translation Eukarya Lophiiformes)
size: 5 ft. to 10 ft.
threat: moderate
This species, despite it's name, is predominantly a plant based creature who resides within the depths of the Felaryan Seas. Weighing in at an average of 200 pounds, it sleeps at the bottoms of the lower areas of the vast waters, a good place to set up a front to catch prey.
The Angler Seaweedle, in order to catch food, will allow four stalks of "seaweed" to float above it's body, while concealing it's mouth in the sand. It will then wait for an unsuspecting prey to swim up and eat the "seaweed". What they don't know, however, is that the seaweed stalks are actually powerful tentacles that will wrap around the prey and drag it to the seaweedle's mouth, which will uncover itself from the sands. Biting into the seaweed will unleash a powerful tranquilizer into the prey's nervous system, which makes devouring the prey much easier.
It's moderate threat level comes from it's diet. Smaller versions of the seaweedle will feast on large plankton, fish, etc, while larger seaweedles will feast on human sized prey and small mermaids. If needed, it will wave its tentacles to resemble fish movement, or use it to lure humans in.
Other than that, as long as you stay away from any suspicious seaweed deposits, the angler will pose little threat.