Good question! The clarification for this wasn't in the Help. The Help have been updated as follows:
The number of K9s and Horses come from the Equipment Checks in the Member Fields Configuration. These Equipment Checks have one special feature. If the description contains the word "K9" or "Horse" and a person checked into a mission has one of these checked, this will increment the "Number of K9s" or "Number of Horses" respectively on the Reports page. This provides an estimate only to the number of animals on scene.