I've purchased shares in a bunch of horses via MyRaceHorse.com. It's a very good system - when the horses win, there's a payout. Of course they have to cover the reserves (food, entry fees, etc.) before paying out cash. So in some cases the winnings end up just going back to prop up the reserves. On the other hand, if the horse doesn't win at al (or does poorly)l, MyRaceHorse does *not* come back for more money - you'll never have to put up money beyond your initial share purchase. And one has to realize, horses are a risky investment - some don't end up going fast, some get hurt, some get sick, etc. That's part of the risk/reward.
It's also a lot of fun to own a horse and see it race (online the last 18 months - maybe in-person soon!) and bet on it and see it win! You also receive a bunch of video updates of the horse - workouts, feeding, resting, etc. Another bonus is to get brief interviews of the jockeys after the race where they explain from the jockey's view how the race went ("she had a lot of stamina left, we just got caught in traffic near the start").
Highly recommended as a fun way to participate in horse racing at modest cost. If you true goal is to make a ton of money, this is probably too risky of a bet to make it worthwhile.