A "Hnickar" is a water "spirit" (or monster, or demon, or creature). The word is Anglo-Saxon and is cognate with a ton of similar terms in Scandanavian and European languages. Water lilies are nøkkeroser (Hnickar roses), in Danish, for example. They appear throughout European and world culture. Folkore and traditional culture is often mysterious and defies categories, which is part of its charm, but I'll set down a few characteristics which crop up often.
Everywhere. In the sense that you can find traditional accounts of similar "beings" all over the world. Kappa in Japan, bunyip in Australia.
Associated strongly with water and wasteland/the countryside. In Norway they are believed to dwell in waterfalls, but they can live in rivers, lakes, ponds, wetlands, even ditches.
Intelligent/sentient. They make plans, have internal motivation beyond hunger, and converse with humans they encounter.
Dangerous, but fickle and occasionally benevolent. Most commonly Hnickar are deadly and will kill those who come across them. However, some cultures describe Hnickar offering gifts, knowledge, even marriage to humans if their conditions are met. They can be bargained with in some circumstances, but they are most often murderous.
Shapeshifters. They can appear human/humanoid, or like a beast. Some cultures (Celtic and Scandanavian) claim that Hnickar take the shape of a "water horse" that could appear friendly before dragging the unwary into the Hnickar's home waters to drown.
Let's leave aside the possibility that these supernatural beings really do, or did, exist. They appear to fulfil a specific cultural function, creating aversion to solitary trips through unsafe land, dangerous bodies of water, and strangers. I enjoy the fact that more ancient and traditional cultures chose so uniformly to populate and encode their lands with hidden meaning, maybe as a response to having to live with so much half-formed fear and real, but misconceived danger.