Check with your local planning department on what is allowed. Even if the horse rescue is not a violation, it must be done properly, and should not cause a health issue. So you may also want to check with your environmental health department.
This response is not to be construed as establishing an attorney-client relationship, and provides general information on the subject at hand only.
In your area, the type of use you describe could in fact be allowed. Sometimes, a special permit might be required. Check with your county and ask. They will tell you if it is allowed and if there were any permits obtained.