No person shall own, keep, shelter, or harbor for any reason within the City a dangerous animal so defined herein, except in the following circumstances:
1. Animals under the control of a law enforcement or military agency.
2. The keeping of guard dogs. However, guard dogs must be kept within a structure or fixed enclosure at all times, and any guard dog found at large may be processed as a dangerous animal pursuant to the provisions of Sections 57.05 and 57.06 of this chapter. Any premises guarded by a guard dog shall be prominently posted with a sign containing the wording “guard dog,” “dangerous dog,” or words of similar import, and the owner of such premises shall inform the Chief of Police that a guard dog is on duty at said premises.