That kind of dip in the neck can be a result from a hind hoof being pressed into the neck when in the womb. It does happen more often when there is a big baby and small mommy. Can he winny? Because that dip can also be a result of throat surgery.

My horse Wally had a dip exactly like that, but he couldn't whinny, but he certainly tried- it was a weird throaty sound. So his dip was the result of throat surgery.

imagefunny pictures of cats with captions
more animals