Hell Pig

Archaeotherium
National Museum of Natural History, Smithsonian Institution, Washington DC

This is a type of entelodont, a distant relative of modern pigs. It's odd teeth, though similar to pigs, indicate scavenging on meat playing a larger part of the diet. Though one might not think a pig is the scariest thing in the world, this animal could have looked you in the eye, with a shoulder height of about two meters. They were the apex predators of around the Oligocene-Miocene boundary across the northern hemisphere.

Roofed Lizard

Stegosaurus
National Museum of Natural History, Smithsonian Institution, Washington DC

I'm sure everyone who might read this knows what this thermoregulating, Jurassic ornithischian knows what this looks like, but here goes:

Whorl-tooth

Helicoprion
National Museum of Natural History, Smithsonian Institution, Washington DC

Though it is not clear by looking at it, this is perhaps one of the most interesting vertebrate dentitions. It is actually the teeth of the lower jaw of an ancient shark that lived from 280 to 225 million years ago. The entire set of lower teeth is visible. How it captured prey with this unique structure is not understood. Here is what it would have looked like in life:

And Now, For Something a Bit Older

Triceratops horridus

National Museum of Natural History, Smithsonian Institution