As requested by theLOAD, here is a family tree of all the beings included in the "Nix" family, or how a bunch of water horses, a river dragon, and a violin playing devil are all related to each other. Line-art version because I will never be satisfied with colors here.
Before I get into that, however, I should say that this shouldn't be taken as absolute fact. Firstly, it's absolutely incomplete. There are many, many, many different water horses in Western Europe. I kind of just lumped three of them together in the bottom left corner. One could argue that the Welsh Ceffyl Dŵr should be on here, as well as the Belgian Kludde or the Breton Mourioche. There's a whole lot of them. Secondly, it isn't as cut and dry as the tree makes it seem. The knucker didn't spontaneously turn into a dragon, it was probably influenced by dragon folklore in the area before the name stuck. Outside influence is all over the place. Were water horses in Britain derived from English and Saxon sources or did the occupying Norse bring them? It's unclear. As well, there's probably a lot of internal influencing going on. It's hard to tell when exactly nixies came about. Were they influenced at all by the Scandinavian nøkk? When? Making family trees larger than this is hard. It would be all tangled up. For now though, let's look at the creatures that are on there right now.
The oldest of the bunch are the Old English nicor and Old Norse nykr. I couldn't find much about the nykr specifically, but the Icelandic and Faroese nykur was probably the best bet for what the Norse believed about it. The two share an old, old name, and based on the common characteristics it was probably similar to these two. Both the nicor and nykur are water monsters. That's basically it. Outside of Beowulf, the nicor is mentioned rarely if at all. There it's no different from any mundane water creature, and the word could also be used to refer to hippos as well. It's an ambiguous water monster. If specific characteristics existed, they probably haven't survived to the modern era. Judging the nykr by its Icelandic equivalent the nykur, it was primarily a water horse. A creature which mimics a regular horse standing by a river, beckoning weary travelers and children at play to ride it. Then the riders find themselves stuck as the horse leaps into the water, drowning and devouring the unfortunate soul. The nykur is a shapeshifter, rarely known to take human shape as well. It can be discovered by looking at its hooves, which are backwards.
From there, we split the two into Britain and Continental Europe. Britain has the most regional variants of water horses. They are very much similar to the nykur of before, with some local flair or quirks. The mischievous nuggle of the Shetland Isles has a wheel-shaped tail, while the spiteful, kidnapping tangie can equally appear as an old man covered in seaweed. In Scotland kelpies, the each-uisge, and the Manx glashtyn are all quite similar, being equally water horses while also taking the form of men. Strangely, one branch of the tree led to the knucker, a Sussex water dragon. While certainly different from its cousins, it does bear a resemblance to the ambiguous nicor of yesteryear. Knuckers are slippery fiends with monstrous appetites. The most famous one was killed by feeding it a poisoned meat pie. Knuckers live in knuckerholes, round kettle-like bodies of water that stretch deep into the ground.
Then, continental Europe. Here the water horse still lives, but was overshadowed by its more humanoid forms. Brook horses are found across Scandinavia and mostly act the same as their counterparts in Britain. A regional trait is that some (I believe Danish or Swedish) can extend their bodies if a large group of victims want to ride it. Pretty nightmarish. This horse just extends and then jumps in the water and kills you. The other form is the nøkk* (Norwegian/Danish), neck (Swedish), nakki (Finnish), or fossegrim/strömkarl. This is a man-shaped being that sits in rivers and waterfalls, usually playing a violin. The music lures people into the water, where he then drowns them. Fossegrim is also an accomplished shapeshifter. His usual form is that of a naked young man, but he can also become a brook horse at will, or simply melt into the water. Regardless, there's usually one part of him that signals his true nature (horse hooves, an inhumanly crooked nose, a third eye, hairy legs). While usually malevolent, killing or kidnapping people, some have been able to coax music lessons out of him. He could also act as an omen for drownings. In Germany, the nix or nixie is similar in that they will sometimes use their enchanting voice to lure people to their deaths, usually in rivers and lakes, even the Rhine itself. Nixies can also be pretty chill though. They can equally be male or female and even take human form. The one give away is their sopping wet clothes. And no I did not unintentionally lift the design from the skin deep comic I swear please don't send me to jail.
*(also, language lesson! sometimes you'll see it written as nøkken in english. this isn't a variant of the name, but "nøkk" in its definite form, basically meaning "the nøkk")
