You control at least one additional nontoken creature.
You have at least one creature card in your graveyard.
Whisper does not have summoning sickness.
Steps
Activate Whisper by tapping it, sacrificing Breaching Hippocamp and any other creature you control, targeting any other creature card in your graveyard.
Ashnod the Uncaring triggers, copying Whisper's ability, targeting Breaching Hippocamp.
Resolve the copy of Whisper's ability, returning Breaching Hippocamp from your graveyard to the battlefield.
When Breaching Hippocamp enters the battlefield, it triggers, untapping Whisper.
Resolve the original Whisper ability, returning the targeted creature from your graveyard to the battlefield.