WDT is introduced in Episode 8. It is blatantly NOT linked to a Real Life race; some of the prototype horses could not have raced together for timing reasons.
There are no more Real Life races for Suzuka. But she needs to run for the anime, she promised Spe-chan. Perhaps this race is for her and Spe-chan? El Condor Pasa might also join, Back for the Finale.
Basically "tales of a jockey: the anime", but with girls. His cameo in Episode 5 prompts this speculation.
Both main heroines are horses he rode to their spectacular victories, and he was in most, if not all, races that are animated. Moreover, the eating contest in Episode 6 is a reference to an earlier race where he got disqualified (a self-deprecating joke if this hypothesis is true).
Of course, this, if true, does not mean some part of the anime is a documentary. Rather it might be based on the kind of tale told at a table with a bottle of something good, not necessarily to be trusted in every word, but broadly true and extremely entertaining. Only with girls.
Equestrians are generally known to love telling "horse tales"...
- On the other hand, arguably most things referenced in the anime are out in the open anyway, and a good team could collect them all.
... because it happened to her real life prototype
In the same year , his career and life were cut short by an injury suffered during the running of the 1998 Tenno Sho.
And this is the very race that Trainer mentioned in Episode 4 in an obvious "foreshadowing" scene. But the real life horse was injured and put down.
If the series turns dark, I guess it's at Tenno Sho. And the authors gave a hint (as of Episode 6 quite a few hints) to those who would look up the horse. However, they do not necessarily have to follow reality to the full grim conclusion. The horse had to be put down because there is no real way for a horse to heal from a bone fracture in the legs, nor to live with it. Humans (and therefore humanoid horsegirls) can have crutches or wheelchairs. It remains to be seen, as of Episode 5, what will actually happen.
If Yutaka Take is really involved, how they handle it becomes especially interesting, as he was the one on the back of Suzuka when the tragedy happened.
- Tehho Sho is Episode 7 and Yutaka Take does not appear. The series did not turn dark. Suzuka was hurt and slowed down in a way strongly resembling the video of the real race, but, as we find out at the end of the episode, Spe-chan ran to her and supported her leg, which may have saved her from death. Suzuka is in hospital at the end of the episode.
She starts as an Ill Girl, which corresponds to the prototype's situation in 1998 - he missed out much of the season because of an injury. But he came back in late 1998 and then in 1999 rivaled Special Week in a number of races. It would be logical for the anime Grass Wonder to get a similar change and to occupy the "friendly rival" position that is, as of Episode 5, occupied by El Condor Pasa. Especially if El Condor Pasa leaves for France as her prototype did.
In Episode 6 she does get some attention as she comes back for Mainichi Okan and, as in real life, fails to win.
- With Episode 8, this one is confirmed.
There are no horseboys. And this is not a male-free universe. Trainer (Coach) is male. Special Week meets a man (!) walking his dog when she is out running in Episode 4. But no horseboys are present or hinted at. Nor fathers. (At least as far as Episode 4, when this is written)
A logical explanation seems to be that horsegirls are genetically engineered. The project is ongoing, and a horsegirl (Spe's mother has horse ears in the flashback in Episode 2) is artificially inseminated, in order to produce improved offspring. Perhaps Spe's second mother is actually a scientist who inseminated her birth mother?
The horsegirls live lives devoted to running. No other interests seem available to them; they are in a big cool school, but clubs don't seem to exist, except running teams. Moreover, the running is competitive. And the motto of the school is "Eclipse first, the rest nowhere". It is the motto of a real life racehorse, but it also means that losers are out.
That real life racehorse, Eclipse, is the progenitor of most Thoroughbred horses in existence today. It is logical to assume that the same happens with horsegirls. The best runners are bred. Perhaps their genes are even sliced together? (By the way, their propensity for yuri, played up in the trailer and hinted at with Spe/Suzuka, is especially logical if they never reproduce the natural human way).
A further guess is that the experiment is run by the Yakuza who originally controlled a segment of the idol industry. This would explain the weird "winning concert" tradition - someone right after a big run is usually not the best singer, but this might be a tribute to where this industry originated. (And now I want fan art of that female trainer in a swimsuit, with full irezumi (Yakuza tattoo).
Also, there don't seem to be any adult horsegirls in the races, it's all the schoolgirls, so where do they go? There is also that moment in Episode 4 around 07:50 when Trainer is talking to Suzuka about a few races. And he says "Once the fall Tennoshou is over, you'll..." then falls silent and we see a thoughtful and apparently sad Suzuka. And she is already an established horsegirl. So is that time the moment when she will finish her racing life, and either be bred - or else be eliminated, "sent to the glue factory"?!
According to https://www.raceadvisor.co.uk/happens-horse-retires/ there is a third option for real life racehorses when they are retired. They can be "retrained and rehomed", and so they become pets or are used for equestrian training. But there are no riders to train in Uma Musume world and pet horsegirls would veer into outright hentai territory, so I guess these options don't exist.
So... is this a Crapsaccharine World? There was a suggestion on a forum that the anime is a metaphor of idol life. That could be fitting.
However, after Episode 5, this scenario is looking very unlikely, as a connection to the real life horse racing industry was clearly established with Yutaka Take's cameo.
- Jossed (according to unconfirmed information on Discord) by a Word of God live event shortly before Episode 6 that included a Q&A:
While the history of a world without horses and with horsegirls might be significantly different from our reality, this may have been intentionally left to fanwanking, especially with all the reproduction questions marked as "secret" (unless a reveal is planned in anime or manga or game).
- This is a motto used by the owner of a real racehorse named Eclipse, who is the progenitor of most modern Thoroughbreds.