Thomas the Tank Engine must be the world's most adventurous train. After gracing the worlds of Resident Evil 2, Monster Hunter: World and Fallout 4, he's now rolled into Sekiro: Shadows Die Twice, replacing the Great Serpent in a new mod.
Sekiro's Great Serpent appears several times throughout the game, so you'll have plenty of chances to feast your eyes on Thomas' eerily unblinking face. You don't get to hear his theme tune, sadly, but he will blow an amusingly squeaky whistle just before he attacks.
Don't expect much in the way of animation work: this really is just a replacement for the front portion of the snake, so Thomas will jerk and twist in unnatural ways, but it's good for a laugh or two.
If you're looking for something slightly more serious, check out this mod, which adds on-demand elemental damage to your katana.