That they controlled the boundaries of the arena was fairly obvious, though I think a big wall would have done the job better than a forest fire. When I was watching it happen I couldnt help but wonder what would happen if Katniss had returned to the boundary after the fire had burnt itself out.

Having never read the books and maybe it explains it better there, but on the big screen those mutant dogs very defiantly are shown appearing out of nowhere and exactly where the game controllers wanted right when they needed them to. Its never explained where they came from. Not even a throw away line to explain it. Nothing.
If they were hidden in underground kennel/boxes what harm would it have done to show a the lid flap open and the dogs run out? or have a techie sat something like "we have x dogs nearby, should we release them?"

As its shown, they meld out of the ground as if by magic/uber-tech in a seeming response to a technician creating a hologram of them