Tails adds in a bunch of different tails, ears and wings including various subtypes all of which you can colour to your liking!
It's a primarily client side mod with an (optional) server side component for syncing tails across clients. However even if the server doesn't have the mod installed, other players with the mod can still see your tail! All you have to do is use the fancy in-game editor then export a modified version of your skin (don't worry, we use the unused pixels so there will be no visible change on your skin) and upload that to Minecraft skin servers and bam, other players can see your tail without having the mod server side!
Each tail is unique and different too with custom animations for each one to make them feel alive rather then something just tacked on. In the future, animations will be dynamic to what you are currently doing and react accordingly.
And of course, each part is customisable. Parts can have up to 3 different tints applied to them, with the entire full RGB colour available so that's over 16.7 million different colour choices per tint! Certain parts have subtypes so they are similar to the "main" part but with slight variations such as added parts etc and in the future, you'll also be able to select from different textures per part to make it even more unique to you.
You can open in the in-game editor by opening up the in-game menu (When you press Esc) and clicking "Tails Editor" button in the lower middle. Most of the buttons in the mod will have a tooltip to explain what they do but just have a mess about and see what you can make!
Curious about what tails are gonna be added in the future? Take a gander at this spreadsheet!