56-56-123. (jockey joke.)

Basically Still's jockey Miyupi showed up on a TV show one day to commentate on boat races... where he revealed he bets on boat races quite a bit and that his betting strategy is the infamous 56-56-123, much to the horror of his co-hosts:

It's basically the absolute longest of long shots betting the outermost disadvantaged boats finish first and second.

This led to Uma!Still being associated with boat racing. Either being a boat racer herself to earn her keep (read: sweets budget), being an incorrigible Chrono-tier gambler who bets on boat racing, trying to brainwash her trainer into boat racing and using the 56-56-123 strat (or just waving a sheet of paper with the numbers written on it as though it's a mysterious horror-game string of inexplicable numbers), or pulling a 180 becoming the straight man reacting in horror to her trainer blowing away loads of money on 56-56-123.

So far not a lot of art of this yet. Most of this is in fanfics, or hastily edited together images, or in Umamusume Tanuki videos, or forum roleplay posts.