Ears (+ Snouts/Muzzles, Tails, Horns, Wings, and More)
More skin customization options for just about every version + skin backports/fixes for pre-1.9 versions.
Created due to a lack of the Tails mod for 1.16, Ears is a mod that adds ears, snouts, tails, horns, wings, and more to the player. Eventually, it also became a complete lightweight open source 1.9 skin backport for many old versions, which works even if you aren't using its special features.
Ears has a more vanilla-faithful appearance than Tails, with a lot of 2D regions but a few 3D ones too. There are a wide variety of possible configurations and it can all be customized.
Why use Ears instead of Tails? Because Ears is easy to port to other versions, based on a multi-version abstraction allowing the mod to even run in web browsers. The Tails mod has wonderful 3D models and animations, but it's a burden to update it to any version due to its large rendering system and all the data syncing code, causing it to be tightly coupled with the version of the game and mod loader it was designed for. Ears stores all data as pixels in the player's skin.
However, Ears requires you to draw your own additions, and does not come with easy recolorable prefabs like Tails. Additionally, unlike CPM or Figura, Ears has a limited number of customization options. (Note that if you have a simple idea, I may add it. Open an issue on GitHub or hop in Discord.) Ears offers a good middle-ground that will work in any version you wish to play, and will receive updates to snapshots for those of you who live on the edge. It also does not and never will use external servers, just Mojang's skin server, so there's no concerns about a third-party server going down (be it temporarily or for good).
Why use Ears instead of other skin fixes? Ears provides a lot of new features to customize your skin, and is a much more lightweight patch with no usage of third-party servers and is completely open source. Additionally, Ears backports translucent secondary layer support to 1.8 and 1.7.