1st the dog depends on ur starter
then u must 1st beat the e4 and the machine on isle 1 must have both the saphiregem and ruby gem in it then go walk around Kanto(pallettown-cinnabar-victory road) until u find one i recommend master ball or use a pokemon that has a special tjhat prevents foe from fleeing and the pokemon must know either a move that paralyzes or puts 2 sleep weaken the leg dog then try and catch it