This is a cable tie gun. A cable tie is used to bundle wires together. See http://en.wikipedia.org/wiki/Cable_tie for more information about cable ties. A cable tie gun is simply a tool which makes it easier to fasten a cable tie to a wire bundle, tighten it to the appropriate tension, and then snip off the extra end. Search google for "cable tie gun" to see many other examples of this tool: http://www.google.com/search?q=cable+tie+gun.

I'm trying to understand how you take this for a walk on a leash, seeing that it is in the pet category.

There is a drawing (pdf) here http://tinyurl.com/5y47xf and Operating Manual on the next page for the Panduit GS2B tool

tie gun