If you control Sylvan Hierophant when it goes to the graveyard, you exile the Hierophant and return a creature card from your graveyard to your hand. It doesn't matter whose graveyard the Hierophant goes to.

If Sylvan Hierophant is removed from the graveyard before the ability resolves, you still return the targeted creature card from your graveyard to your hand.

It targets a creature card in the graveyard. This target is selected when putting the triggered ability on the stack. The wording specifies it has to be a card other than the Hierophant.