The Sims 3 Pets introduces three new skills to the game: Riding for Sims and Racing and Jumping for Horses. These three skills come together to measure the skill of horse and rider. Maximizing these three equestrian skills will give your Sim access to more profitable stud services and horse sales. They'll also be more likely to place well in the new riding competitions. This is one way to make money for your Sim's horseman self-employment career. Riding is not necessary to adopt a Unicorn, but these magical steeds start with max Racing and Jumping skills.
Learning the Skills
You need a Sim and an Adult Horse or Unicorn to get started. Simply riding around will start building both the Sim's Riding and the Horse's Racing skills. Riding classes are available at the Equestrian Center for $500. The advanced jumping obstacles are the best way to train a horse, as you will gain riding, racing, and jumping all at once. Horses will get fatigued the harder you push them, but this generally leads to faster skill gains overall.
Horses are more valuable when sold at max skills. Every level gained will bump the price a bit. Completing the racing and jumping skill challenges that impact value will allow the horse to make a more valuable stud or mare.
Sim Riding Skill
The Riding skill represents your Sim's ability to control the horse and go with its movements. This skill is the single most powerful, giving Sims new options as they progress. Horses will only unlock new competition types upon gaining levels. Doing anything on a Horse will raise Riding, even simply being mounted for a time. Here are the various things you will unlock as you level your Sim's Riding skill:
Level 3: Can enter Show Jumping Competitions
Level 5: Bareback riding
Level 6: Cross-Country Competitions
Level 7: Heroic Pose interaction while mounted
Level 8: Sim can adopt wild horses with sufficient friendship
Level 10: Sims can whisper to horses around Acquaintance, a powerful social interaction for befriending wild horses and Unicorns.
Horse Racing Skill
Your Sim can train a Horse's racing using training posts. Set them up so that the posts form a loops. These are just guides, and can be very well spaced and still function. You can make a large track this way, building the Racing and Riding skills while training there. At level two, the Horse will help to raise the Sim's Riding skill a little faster.
This skill has a heavy impact on racing competitions, and a partial impact on cross-country.
Horse Show Jumping Skill
The Jumping skill can be trained using two grades of obstacle. The beginner quality are the standard Jump Trainers. Use this to get the horse comfortable with jumping. Anything higher than this qualifies as an advanced obstacle, and will raise the Horse's racing skill in addition to jumping.
You can make jumping courses with the trainers. This will let you set up multiple jumps. Advanced obstacles can be knocked over, and horses will occasionally be too skittish to jump. If you give them more training before starting the big jumps, this will be less likely to happen. To link training jumps into a course, place the next jump on top of the green area indicated in buy mode. Make sure absolutely nothing is in the way and that the green area around the jump obstacle is fully located on your lot. To see a good example, look at the Equestrian Training Grounds in town.
This skill has a heavy impact on Show Jumping Competitions and a partial impact on Cross-Country Competitions.
Equestrian Center Competitions
You can enter competitions at the Equestrian Center at the following hours: 5-9PM Mon-Sat and 12-5PM Sunday.
When racing, you are given multiple racing strategies to choose from: go for broke, take risks, and steady pace. In general, the only competitions that are hard are Internationals. Horse and Rider should be level 10 before entering. Use Take Risks or Go for Broke to gain position then hit steady pace once you've got first. Luck is a factor. If you can place at least third in international races you can win enough that it is better than advanced, so go for it at that point.
Here is a table of the prize winnings that can be made from first place in each of the types of competition: