Originally Posted by PaintedFury
The little bit of crest in that pic is nothing to worry about. It's when they get so much of a crest that it's just huge, and affects their throatlatch that it becomes a problem.
That to me sounds like discounting neurological probs if they fall short of paraplegia!! It is indicative of long term & unaddressed problems when it becomes that severe that it falls over or affects the throat. You would also be likely to see hoof & other health probs way before it got to that extreme. Agree with Nova and Jaydee. Yes OP, you are correct to be concerned.
That the horse was thin(assuming), has a history of founder & a crest despite having been underweight screams insulin resistance to me, of which it sounds like your vet may not be not knowledgeable about. Ecirhorse.com is one great online resource for learning about this.