As long as you control The Goblin Mastermind or it's your commander, permanents you control are Kindred Goblins in addition to their other types. The same is true for spells you control and cards you own that aren't on the battlefield.
Whenever The Goblin Mastermind deals combat damage to a player, you create a token that's a copy of one of Skirk Prospector, Goblin Lackey, Goblin Piledriver, Goblin Warchief, Boggart Harbinger, or Goblin Ringleader, chosen at random.