Alright, first off, after noticing a common motif, created a new tag – offering_collar (not to be confused with offering_leash) and went through most posts tagged with collar trying to tag as many fitting images I could (skipped flash submission and comics, for now). I'm still not certain about the name of the tag itself, so some suggestions would be great, perhaps presenting_collar or holding_collar would be more fitting. Also, probably should create a offering_collar –> holding_object implication.
e.g. post #314867 post #1408893 post #1037631
So, uh, yeah, please use it.
While tagging stuff I also noticed two somewhat underused (and messily used) tags that are both similar to one another – walkies and offering_leash. Tagged a bunch of posts with these while I was at it as well (I only started used this much later in the process, so the latest 15k collar posts didn't get the treatment, if someone is willing to fix that, it'd be great). Pretty sure both of those should be merged into one as there does not seem to be any defining characteristic for either one. Another problem is that there are some pretty tenuous uses of this tag – for example post #1255331 is fairly clear, however post #478569 or post #740848 is not that straight-forward.
Updated