Umamusume OC Guide by readysteadyderby:
Umamusume OC Guide by Orca8989:
Gym Attire Guide by HARUURARA_mochi
Additionally, I say do some quizzes to see what you get for a horse breed. Even if you don't include their breed in the bio, it's still fun to maybe figure it out for those who want to!! I'm partial to uQuiz, so... here's a few links:
Which Uma Musume Character are You???
Which Horse are You? (Umamusume)
What horse breed are you?
What breed of horse are you
What Horse Breed Suits You?
What Horse Breed are You?
What Breed of Horse are You?
What breed of horse are you?
What horse breed is best for you?
What horse breed are you most like?
RACEHORSE OR NO RACEHORSE
If you want it to be an irl horse, I say determine if the horse you wanna use is an actual racehorse, or if it's a horse you know (and yes this includes knowing the name).
If it's going to be an actual, irl racehorse... definitely take into account the jockey as well. Take into account their uniform colors, the horse's pelt colors, both of their personalities, etc. as stated in the images above. Don't skimp the details, and if they have a track history of wins and losses, definitely include those too.
If you're going to use a horse you know about that has no ties to races, similarly to myself with making umas based on my grandpa's horses, take into account their foal-hood or filly-hood. Analyze their personality and how they behave. If you know the owner, maybe even look into them as well to incorporate traits of the owner. If the horse is of non-Japanese origins, such as Western, English, Norwegian, etc. then perhaps even take into account the scenery of the area in which they live, particularly using this in their racing uniform.
Taking the two points above into account, what I say is the most important above all else, is that you represent that horse with respect and love. The canon umas are done with the utmost care and attention, so truthfully you should do the same. And while I know people will... do what they will... with their fanmade characters for any fandom, but... perhaps include the rule for others that no R18, gore, or fetish content be made of them?? Especially if it's made in the image of an irl horse??? A fanmade uma based on no real, living animal is one thing in my personal opinion because you aren't harming the image and reputation of anyone or anything, but... if you're using an irl horse, be it an actual racehorse or otherwise, making that sort of content of the uma counterpart just feels wrong.
Personally, I say save references of the official Uma. Save official artwork, be it character busts, chibis, their outfit references, etc. Below are two links that I feel should help:
Danbooru Umamusume Tag (combined with official artwork tag)
Settei Dreams Gallery for Umamusume Anime
JRA // NRA // JAPANESE RACES
Japanese Racing Association
National Racing Association
Japanese Triple Crown of Thoroughbred Racing
Asahi Hai Futurity Stakes
ᴬʳᵗʷᵒʳᵏ ᵇʸ ᵍᵒᵐᵃˢʰᶦᵒ ⁽ᵍᵒᵐᵃ ᶠᵉᵉᵗ⁾
