Well we (by 'we' I mean DH) called the sherrif's office (we're out of the city limits, so police can't help us), and then I talked to them.. our next step is to contact animal control to report a dangerous dog (or 5), which we have to do tomorrow morning since they aren't open this late.

Honestly the damages are probably NOT worth trying to go take the guy to court or contacting a lawyer - it really does look like the rear derailleur is the worst of it... and it's a tiagra. If I have to buy another to replace it, it's not the end of the world... mostly frustating in regards to the dog.