I had a cooler and leash for Tugger when I was training him to stay near the barn, but I have not used a leash or harness with the house cats. The barn cats always had collars with identification, but it was hard as they lost them often. I had to replace many collars and many RV tags over the years. I don't even bother with Beau and Thomas.
Love All Life, Thank You For Posting! :)