Looking at that hippo skull reminds me of what people who've been in contact with them have had to deal with.  They're actually quite aggressive.  Victims have had their arms ripped out of their sockets by them. 

Of particular note for me is the mythology of ancient Egypt.  Set is described as having a hippopotamus's face.  His signal act was to dismember his brother Horus after killing him (mama Isis made him better).  Pretty much what an enraged hippo does, so you could guess why the Egyptians used that beast as the death god's totem.

I do wonder about the dinosaurs, though.  This shrink-wrapped lizard face is quite persistent.  I recall seeing a picture of a triceratops with its frill hidden behind a mass of muscles that made it look more like a pig than the familiar form we've been given.  Kind of freaky.

But further given their vegetarian diet, the frill serving as the mounting base for strong jaw muscles is not an extraordinary concept. As I have noted before, when this has come up, there are muscle attachment points on a human skull that correspond to those on most other animals, a placing the fingertips on the side of the head, above the ears, and clenching the teeth quickly demonstrates how high our own jaw muscles are attached. Needing an even greater mechanical advantage in order to deal with tough vegetation could easily have led to the development of the frill.

(The black and white line drawing, seen above, does NOT reflect this configuration, of course. There we see a fairly conventional restoration with a "blob" of muscle in front of the frill that serves no apparent purpose.)