Pokemon Legends: Arceus appears to feature some new and subtle changes to various Pokemon. Yesterday, the Internet caught on fire when The Pokemon Company revealed that Pokemon Legends: Arceus would feature new Pokemon and new regional-variant Pokemon. The two new Pokemon shown during the trailer, Wyrdeer and Basculegion, are both evolutions of existing Pokemon and seemed to be nods back to Pokemon Diamond and Pearl, which featured several new evolutions of previously known Pokemon. And while yesterday's trailer also revealed Hisuian versions of Growlithe and Braviary, it also showed off a third Pokemon with a previously unseen form. The Wurmple shown in the trailer has a distinctively yellow horn on its back. In previous Pokemon games, Wurmple's back horns are the same reddish color as the rest of its body.
There are a few possibilities as to why Wurmple has a subtly different look than in past games. The first explanation is that Pokemon Legends: Arceus will introduce new forms to the franchise, similar to how Pokemon Diamond and Pearl introduced the concept of gender differences to the franchise. During a deep dive into Pokemon Legends: Arceus, The Pokemon Company showed off several research tasks related to the Pokemon Shinx. One of these research tasks was to "register different forms." Shinx has two different forms based on whether its a male or a female, but some Pokemon look identical no matter what gender it is. Pokemon Legends: Arceus could be adding additional new gender-based forms to the game as a nod back to the original Gen IV games.
Another possibility is that Game Freak and Creatures Inc. (the co-owner of The Pokemon Company that's responsible for creating the actual Pokemon models used in the game) is using Pokemon Legends: Arceus to update certain models and visual assets. This wouldn't be the first time that The Pokemon Company has updated official models and visual assets for some Pokemon, but it's rare to see updates occur mid-generation.
One other explanation is that the Wurmple horn is just a visual error of some kind. After all, the game footage seen in the trailer isn't final, and it could just be some sort of mistake or visual glitch that will get fixed prior to release.
We likely won't get full answers as to just how many changes are being made to existing Pokemon in Pokemon Legends: Arceus until the game is released on January 28, 2022.