Because Mallow is destined to appear early on in the anime, they had to design her very early. But starter evolutions don't need to appear right away, so they can be finalized a little later. This goes back to what I said a bit before: there is a chance--maybe a slim one, but a chance--that some of these starter evo concept arts are of alpha/beta-level brainstorming. They already had Mallow designed because she needed to be designed earlier, and they just wanted to put her in this concept piece for size reference and setting the scene. Ash's hat being wrong lends credibility to the idea that at the very least, these are super-early concept works, because changing the design of Ash's hat in the anime is minor enough that they could do it a little later (then again, his entire outfit looks wrong but since it's just an outfit change that could also be done later, lending even more credibility to the idea that these works were made very early on).
If they're real, though, and we do get day/night-based split evolutions, I hope they're the nighttime evolutions and we can get a quadruped tiger for Litten, a... less entertainer-seal(???) for Popplio, and a burrowing owl (?!) for Rowlett. If they turn out to be the daytime ones then I'm fine with that, too. Just...
Please Litten. Please stay on all fours.
Please...
