You still put the top creature card from your graveyard onto the battlefield even if this card is not in your graveyard when the triggered ability resolves.

The last thing that happens to a resolving instant or sorcery spell is that it's put into its owner's graveyard. —Example: You cast Wrath of God. All creatures on the battlefield are destroyed. You arrange all the cards put into your graveyard this way in any order you want. The other players in the game do the same to the cards that are put into their graveyards. Then you put Wrath of God into your graveyard, on top of the other cards.

If you control Mistmoon Griffin when it goes to the graveyard, you exile the Griffin and return the top creature card from your graveyard to the battlefield. It doesn't matter whose graveyard the Griffin goes to.

The "top" card of your graveyard is the card that was put there most recently.

If an effect or rule puts two or more cards into the same graveyard at the same time, the owner of those cards may arrange them in any order.

Say you're the owner of both a permanent and an Aura that's attached to it. If both the permanent and the Aura are destroyed at the same time (by Akroma's Vengeance, for example), you decide the order they're put into your graveyard. If just the enchanted permanent is destroyed, it's put into your graveyard first. Then, after state-based actions are checked, the Aura (which is no longer attached to anything) is put into your graveyard on top of it.

Players may not rearrange the cards in their graveyards. This is a little-known rule because new cards that care about graveyard order haven't been printed in years.