Who are the “Outside” dogs? What eventually happens to these dogs, and why?

The "outside" dogs died of starvation.

The first to go was Dub. Poor blundering thief that he was, always getting caught and punished, he had none the less been a faithful worker. His wrenched shoulder-blade, untreated and unrested, went from bad to worse, till finally Hal shot him with the big Colt’s revolver. It is a saying of the country that an Outside dog starves to death on the ration of the husky, so the six Outside dogs under Buck could do no less than die on half the ration of the husky. The Newfoundland went first, followed by the three short-haired pointers, the two mongrels hanging more grittily on to life, but going in the end.