I know the one you mean; I can see the dog now...but I don't remember which monument it's on (or near). I tried to find it in Stephen Recker's "Virtual Gettysburg" database but can't seem to find the right combination to bring it up. If you want to try, it's here:
http://www.virtualgettysburg.com/exhibit/monuments/
Seems to me it was the 69th NY but can't find it.
Alice