Okay, finally something I know. Ricky has had a harness from day one. Lucy Tu has a pink one now. Luckily they adjust (the harness that is not necessary the pups). Ricky's first one still fits him. Anyway, Ricky's Vet told me from the very beginning to get a harness, never let him out without a leash and always be alert.
The harness is so I can pull him up to me quickly in case a big dog is loose in the neighborhood and rushes up to him. Our daughter lost her chihuahua when a big dog took one large bite, broke most of the bones in his little body. HARNESS! I also keep Ricky on a short leash. I have had loose dogs come around a car all of a sudden and if I didn't have him close enough, I might not have had time to get him to me. Remember Chihuahuas don't know they are small and they don't back down. Ricky has even tried to start a fight with a Pit Bull.