Sekiro's modding scene is really taking off: more than 50 mods have made their way to the game's Nexus Mods page in the past week, with more coming on every day. One of the most popular at the moment is modder ZullieTheWitch's costume pack mod, which adds 25 reskins for Sekiro, transforming him into a samurai, Lady Emma, or a shrieking monkey.
The character models are all taken from NPCs and objects in the game. My favourite is the giant lizard with bulging eyes, which is presumably a stretched version of the poisonous geckos you occasionally come across. I also like the one described by ZullieTheWitch as "man with a basket on his head". It looks exactly as you'd expect—I reckon it should come with an added stealth bonus.
Check out the video at the top of this post to see them in action—the animations all transfer remarkably well. ZullieTheWitch says they might add new costumes in the future, or replace some in the existing pack.
As with other mods, you'll have to install the Sekiro Mod Engine first. Once you've done that, you can add individual costumes to your game by downloading the pack from its Nexus Page.