The mod swaps out the Tree Sentinel's trusty steed for the bright cartoon train, which looks as laughably ridiculous as you'd expect.
What's more, judging by skymoon's video of the mod, you can also swap Thomas in for the player's horse Torrent.
It sure is weird seeing Thomas double jumping around Miyazaki's dark fantasy world.
It's a bit of a complicated mod to install however. As PC Gamer explains, you'll need to use a modded version of UXM to unpack the game, then unzip the Thomas mod archive into the Elden Ring folder.
And it seems this may no longer be possible: as skymoon explains on the mod page, "UXM for Elden Ring is not allowed to upload to nexusmods now. sorry for inconvenience. I will update later."
Still, riding Thomas in Elden Ring was an inevitable fantasy, wasn't it?