There's a few varieties of horsegirls though. You're thinking of horsey girl hobbyists. The "catlady horsegirl" is a good analogy for them, they'll have a close personal relationship with their equine pets, feeding and caring for them.
Racehorse ownership is not like that. Usually ownership of the horses will be split amongst a group, called a syndicate. The syndicate pays a trainer to try make a race winner for them. Racehorsegirl won't establish a working partnership with her horse, she'll never ride the horse or feed and care for the horse.
She just wants it to win, wants it to be the fastest horse. Being part of a racehorse syndicate is probably more akin to being part of a bike or car racing ownership syndicate than a hobbyist catlady horsegirl. It's about owning the fastest on the track, and gambling and drinking and dissecting the odds to make sure you win the gambling.
While other gamblers eg casino goers are pretty stupid, on average the house always wins, and so they on average will lose, the deck isn't stacked against racehorse girl's bets. If a racehorse girl is smart enough to study the form closely, along with taking advice and tips from the people who train and ride her horses, she'll likely win the majority of her bets on race day.
Generally it's men who are into this though, as with racing car owners, most syndicates of racehorse owners are male. Being part of a syndicate isn't as financially draining as owning a hobbyist horse (especially if you have a group 1 winner). Although as with most things the more you put in the more you get out, paying a premium for a good horse and a good trainer will yield better winnings than cut price horses and trainers can.
TLDR, Racehorse girl is probably more like a formula 1 enthusiast girl than a catlady horsegirl.
Source: Am female, have considered in the past buying into a racehorse syndicate. I have no interest in ever riding a horse myself, or looking after it. I just enjoy a day at the races, occasionally enjoy gambling where the odds are somewhat in my favour. I like the discipline of studying form, and I like to win on the basis that my studying technique is solid and my advice and tips are similarly good quality. And for big races you dress up and drink good wines and champagne, which is also a very appealing aspect of the races for many women.