Does a creature exiled with Fiend Hunter return immediately if Fiend Hunter is sacrificed in response to its own trigger?

This is a classic 'blink' or 'sacrifice in response' scenario involving a two-triggered-ability interaction. Fiend Hunter has an enters-the-battlefield (ETB) trigger that exiles a creature, and a leaves-the-battlefield (LTB) trigger that returns the exiled creature. The key is understanding how the stack and trigger timing work together.

When Fiend Hunter enters the battlefield, its ETB trigger goes on the stack. If you sacrifice Fiend Hunter before that trigger resolves, its LTB trigger also goes on the stack — now on top of the ETB trigger. The LTB trigger resolves first, trying to return the exiled creature, but nothing has been exiled yet, so nothing happens. Then the ETB trigger resolves and exiles the target creature — permanently, because there is no longer a Fiend Hunter on the battlefield to generate a future LTB trigger (CR 603.6, 400.7).

However, if the ETB trigger has already resolved (the creature is already exiled) and then Fiend Hunter is sacrificed, the LTB trigger goes on the stack and resolves normally, returning the exiled creature immediately (CR 603.6a).

Concrete example: Fiend Hunter enters targeting your opponent's Grave Titan. Before the ETB trigger resolves, you sacrifice Fiend Hunter to a Carrion Feeder. The LTB trigger goes on the stack above the ETB trigger. LTB resolves — nothing to return. ETB resolves — Grave Titan is exiled forever. Opponent never gets it back.

So the answer depends on timing: sacrificing Fiend Hunter before its ETB trigger resolves causes the target to be permanently exiled. Sacrificing it after the ETB trigger resolves causes the creature to return immediately via the LTB trigger (CR 603.2, 603.6, 400.7).