Hello @kbandalusians,
Welcome to the Community. I'd be happy to provide information about options for tracking individual costs for the horses.
The suggestion that @Anonymous provided for using the Projects feature to track expenses for the job could certainly be beneficial. The following article contains additional information on this feature: QuickBooks Online Projects.
Another option that you have is to utilize Class or Location tracking, which is designed to monitor different segments of a business. This would have you assigning a location or class to the horses to run reports with, such as a Profit and Loss filtered by class/location. Additionally, these reports can be filtered so that only expense transactions are included, so that you can find out how much each horse is costing individually. Please be aware that Class and Location tracking is currently only available in QuickBooks Online Plus.
I'm including some links with further details on this feature:
As is always the case, you may wish to reach out to your accountant to ensure the most optimum setup for your business.
If you should need anything else, the Community is always here. Wishing you and your business continued success.