Around 1km east of Wave Rock is Hippo's Yawn, a granite cave eroded from the base of a big orange boulder, which does indeed resemble the gaping jaws of a Hippopotamus amphibius. Walk into the maw and imagine you're something's lunch. Parking and access here incur the same costs as at Wave Rock; pay your fees there before walking or driving here.
Hippo's Yawn
West Coast Australia