From New York Magazine, "Dogs Looking Depressed in Their Halloween Costumes":

Photo slideshow of the annual Halloween dog costume extravaganza at Tompkins Park, with dogs looking understandably pissed off:

http://nymag.com/daily/intel/2010/10/dog_halloween_parade.html?%3Fmid=facebook_nymag#photo=1x67315

To each their own and all that, but I'd never dress up Thomas because he's already one of history's cutest creatures and it'd just be gilding the lily. Plus, as the pictorial demonstrates, dogs don't want to wear your grandfather's fedora.