Do creatures from outside the game go to the graveyard?
Once a card has been brought into the game, it is treated like any card you have started the game with.

The game rules make no distinction between cards from your deck and those that have been brought into the game after the game has started, they function exactly the same. Therefore, any card you bring into the game with  Spawnsire will do whatever it usually does, including going to the graveyard if the game rules require it.

The only difference is that you have to remove those cards from your deck before the next game starts, but that's a matter of tournament rules rather than game/match rules.