In order to actually enter, you will also have to tell your jockey how to run your horse in the race. The way a horse runs in a race is referred to as racing style. There are three styles available, and each style focuses on two of the base stats (check out the other guides for more info on stats). All 6 stats are added in when a horse races, but whichever two correlate to the style it runs as will be doubled. Therefore, you should always try to run a horse on a style that uses the two highest stats possible (or the highest combination).
Frontrunner: These types of horses break fast and like to get the lead early in the race. They usually set fast fractions, and may pull ahead to a wide lead early in the race.
Midpack: These horses won't want to be on the early lead, but will want to be right behind it. They may lag a little early, but never want to fall behind too far. They'll usually be in a larger pack of horses, so they need to be able to handle a crowd and will need to be able to trust their rider to guide them in between and around other horses.
Closer: These horses are sluggish early in the race and may fall to well behind the pace. Sometimes they may even seem out of the race entirely. However, they will perk back up at the end of the race and usually make their move late (sometimes very late). They'll be the horses to have you on the edge of your seat as it comes down to a tight finish!
Racing Style Vs. Style Preference
If you have forever pro, you will be able to see what your horses's actual pace preference is. On occasion, it may be in your best interest to race your horse at a style other than the one that uses their highest stat. In general, if the difference between their highest stats and their preferred style is less than 3, you may want to run them in the style opposite to their pace preference, to try to force the pace of the race into the one they prefer. If a race has a bunch of closers in it, it will have a slow pace. If it has many frontrunners/midpack racers, it may end up being a fast-paced race. Generally speaking, there will be far more slow-paced races than fast-paced ones, since it takes quite a few frontrunners/midpack to create a fast pace.