1 - Helmet Shape
Hisuian's helmet differs from Unovan's in that it covers Samurott's eyebrows, goes down below the eye level and stick out of the snout a little bit:
These, however, don't seem to be the case in the sprite, as its eyebrow is still visible, the "wavy" helmet shape is not present and the part that covers the snout is hardly noticeable, not really spiking outwards like the design originally does but rather sticking to the snout:
2 - Some Colours
Ok so firstly, I know this might come off as a nitpick, but the colours on the eye and on the body are slightly off. Putting them side by side for comparison's sake:
It may not seem like it at first, but consider comparing both the sprites of both forms as well as both the home renders and their respective models:
In the sprite, Samurott's body colour and eye colour are nearly the same between both forms. However, in the art, models and HOME render, Hisuian's body is a darker and less saturated shade of blue, with much redder, more vibrant eyes. The sprite currently doesn't take either of those into account, instead sticking to nearly the same colours as Unovan.
3 - General Details
This covers 3 different points that aren't really related to one another but had nowhere else to be covered. First and a rather simple one: Hisuian Samurott has an exposed fang when its mouth is closed, which the current sprite lacks:
Second, Hisuian Samurott's horn points much more upwards than its Unovan counterpart, which is also not currently well represented within the sprite:
Lastly, Hisuian Samurott's body is slimmer than Unovan's, being a lot less bulky when compared to side by side, having a thinner body and limbs, as well as a tail that isn't as wide, which is specially noticeable in the models:
Right now, the Hisuian sprite has the same proportions as Unovan Samurott, which is innacurate.
Now, to illustrate my point better, I've attempted to edit the sprite - Don't get me wrong though, I merely made this just to display my points, I am in no way implying this edit to be used, as I'm not a part of the sprite project or anything like that and I am sure the sprite team could do a much better job than me at applying these fixes to the design. Take this merely as visual reference, sort of what direction I personally believe the sprites should take in order to make the sprite stay closer to the original model:
Old vs My Reinterpretation
As I've said, this is merely visual reference so I can communicate better how I believe the sprite can be improved upon. The idea with the edited sprite is merely so I can give you guys an idea of where to take the sprite, I'm sure the spriters could do a better job than my edit (Personally I don't think I did a great job slimming it down, as I missed out on the limbs and tail; and the horn could use some work, for instance).
That is all! Hope my points didn't come across as confusing or nitpicky. Thank you if you read up til this point!