I would like to see a Water-horse based on the Kelpie/Neck(Nøkk)/Each-uisge/etc.
I think it could be a great way to explore and redeem the Nuckelavee a bit by showing us more of what its component parts could do and giving the horse part some special abilities. The Water horses I listed above are especially useful for basing a horse Grimm on because like the Nuckelavee they come from a sort of mix of Scottish and norse folklore and legend. The Neck and Nøkk even sound similar, Neck>Nuck>Nuckelavee. Also these kinds of water horses typically have the ability to make anyone who rides them stuck so that they can drown them by riding into the sea. Not something that would make sense as a special ability in RWBY but its another thematic connection to the Imp grimm having become fused stuck to the horse grimm to create a Nuckelavee. It could also explain how Nuckelavee's are created to begin with, like if a Imp rides a Neck when it goes underwater they become fused into one greater Grimm. Another good fit is the fact that the Nøkks are known for their screams, and we all remember how chilling the Nuckelavee's scream could be:
The nøkk was also an omen for drowning accidents. He would scream at a particular spot in a lake or river, in a way reminiscent of the loon, and on that spot, a fatality would later take place. He was also said to cause drownings
So thematic stuff aside, what would a Grimm based on these critters bring to the table? Well to begin with they are amphibious and often live beneath lakes, rivers, waterfalls or even in the ocean. Again this makes them fit very well together with the Nuckelavee, which was also a ocean based creature. So add amphibious capability to the Neck (and by extension, the Nuckelavee's) abilities. Its a understated sort of ability, not something that would have popped up in the RNJR fight, but on the whole its pretty useful. It vastly extends the Nuck's reach, as it can travel under lakes and rivers, as well as in the ocean, which will both let it hit targets that would normally be inaccessible (such as villages on islands or traveling ships) but also hide itself from retaliation by going to the bottom of the sea.
They are typically also shapeshifters, often turning into handsome men to try to lure maidens to their doom, and while that wouldn't fit well as a Grimm ability it can be if we modify it a bit. See according to legend the Nuckelavee is only horse like when its on land, it takes on another form when its under water, though no one knows what that was supposed to be. Again this is something that wouldn't have come up in the RNJR fight but which could easily fit if its retconned in. So when under water the Necks and Nuckelavee can shapeshift into some other kind of horror that is optimized for underwater combat, maybe a great big serpent with a horse skull in place of its head or some kind of octopus-centaur.
Finally the ability that interests me the most and which could turn the water horse Grimm and by extension the Nuckelavee from being just another big strong Grimm to something that could potentially solo large settlements is this particular tidbit about the Nøkks:
The Scandinavian näcken, näkki, nøkk were male water spirits who played enchanted songs on the violin, luring women and children to drown in lakes or streams.
They're basically horse Sirens and this is an ability that could make them terrifying against settlements. A recurring idea I've had is that I would like to see Grimm with mind affecting abilities, like Sirens with enchanting songs, peacocks with a dazing/stunning type ability when they reveal their tails, lions or manticores with fear inducing roars, etc. And a common thread for these types of ability would be that a active and decently strong Aura defense can ward them off, making them of little use against Huntsmen and strong warriors, but great at dealing with large swathes of mooks and civilians.
Basically with this ability I envision that the Nuckelavee could have destroyed Oniyuri by itself because the rich idiots who made it spent all their cash on hiring tons of mooks and big guns to defend the place and didn't bother with Huntsmen because they thought them to be overpriced and unnecessary. Cue the Nuckelavee wandering in and mind whammying all the inexperienced defenders and then either ripping them apart or luring them into a lake to drown before coming down on the now defenseless town.
Sadly I cant think of any way to tie the Nuckelavee's poison breath ability into the Water horse myths, none of them seem to have abilities like that unfortunately.