Just personal scratch.
Advised, possible spoilers.
Opinion & Informal
Art: 6/10 CGI characters with "photo" backgrounds. Anatomy not particularly believable. (Serval has two sets of ears, but I actually prefer that lol. It feels like she would be weird if it got technical and she didn't have her human ears and only her animal ears). CGI rendering and backgrounds were noticeably on the less-sharp side. New character models used for the horse-Friends that were introduced in this. Same models for the main characters. Backgrounds were "photos" of a horse derby building and track.
Animation: 5/10 Minimal movements. Used a few single-frame scenes which can be mostly overlooked considering this
is a 3-minute spin off that doesn't really add to the main series at all. It was more along the lines of a promotional piece. Decided not to animate the race between them which is a minus, but that totally would have gone over budget.
Characters: 6/10 The best characters in the buisness. I'm just kidding. I just personally like Kemono Friends so I'm glad that this addition was able to add a little more life to two of my favorite characters. Objectively though this ONA did nothing to add to any of the characters as a whole, development or lore-wise. This ONA failed to be meaningful in any way to their growth or being (It did make Serval really tired though if you want to count the race and epilogue scene).
Plot: 4/10 Spin-off promotional piece of sorts.
Development: 5/10 Showcases some lightly dressed horse-Friends which I personally disagree with as most Friends from the main series are much more modestly dressed. Objectively, zero development to the main series plot. The two MCs find a horse derby. Meet Friends. Race.
Music: 5/10 Uses the same bgm and soundboard from the main series; nothing new. No particular sound design to note in this ONA.
Conclusion: 4/10 Did not animate the race which was supposed to be the climax of this ONA. Skips to the epilogue scene after presenting the promotion's advertisement bit.
Overall: 5/10 Didn't add or detract from the plot or characters. Promotional piece. A nice little slice of "extra" to add to the Kemono Friends world.