I'll try to organize this xd
I'll just be listing and maybe talk about all the ideas and plot elements that I decided to scrap from the fanfic. Some were in order to follow the vibe the canon uma musume series have.

Unused uma names for the ML (Parallel Flash):
-Black Flash
-Ghost (later found a PH horse already has this name xd)
-Black Phantom
-Multo (Filipino for Ghost)
-John Doe
-Parallel Pandora
-Love Pandora
-Pandora Lover
-Wild Sky (Sick name. I might use this for an oc uma later.)

Scrapped Portions of ML's backstory:
-ML is the grandchild of a shaman or fortune teller who constantly demeans ML by calling him a cursed star. Because of this, ML's parents can't help but be cautious around him even though they love him. 
-ML's family has him wears girly clothes during childhood.
-ML is isekai'd by getting stomped to death by a rogue racehorse at a race track.

Scrapped Ideas:
-ML becomes part of Team Spica, but since they all think ML is a woman, he has to constantly sneak away whenever he has to change clothes.
-ML becomes the trainee of a female OC trainer and they fall in love and do the shaggy shaggy.

[Scrapped World Plot Elements]
>"Long ago, uma musume were like Amazons who formed tribes far from human settlements and they frequently kidnapped human men." and not the wholesome kind.
>"Human men evolved to have greater stamina and sturdier bodies to survive getting pounded by both evolved human women and uma musume." This one isn't so much as "scrapped" as more like I didn't clarify xd
>Historical conflicts of uma musume and human women.
-Human soldiers ambush exhausted umamusume right after they finished a race.
-Umamusume ambush human soldiers mid-bathe in a river.
-Either side gifts a giant wooden dong statue to the other. Reference to the trojan horse.
-A legendary conqueror uma musume who unified several uma musume tribes, and abducted and turned over 50 human men into her consorts and had nearly a hundred children.
>"Long ago, human girls born into an uma musume tribe were... not treated well. Human boys born into uma tribes were 'made use of.'" I don't wanna go into detail. I leave it to your guys's imagination.
>"Perhaps as an evolutionary choice due to centuries of conflict, less and less boys were being born in favor of more able bodied fighters i.e. girls. In the past, the ratio was as bad as 9:1 female to male. Realizing this fact, uma musumes and humanity begrudgingly formed a truce. The two races had a rough start due to extreme racism, but slowly fizzled out when women of both sides agreed on one thing, "We love banging men.""
>Details on the truce:
-For both sides, forcing unconsenting men is outlawed.
-Criminals will be judged by courts entirely comprised of their respective race. Umas judge umas, humans judge humans.
-Slavery of either race is highly illegal. Offenders will be executed.
-Child abuse and filicide is regarded on the same level as slavery. Unwanted children of either race are encouraged to be sent to orphanages.
-99% of the modern generation isn't aware of the truce and merely view the details as common sense.