On each horse's profile, you can assign the default work gear that each horse wears or carriers during track work. 

Navigate to the horse profile then click the edit pencil in the Work Gear box.

Select any relevant gear from the drop down Selected Gear field.

When adding track work, this gear will be applied on the track work task by default. You may choose to remove or modify the gear for each track work task, however.

You may wish to review: How to configure Trackwork