Cresty necks aren't just from being fat. They can have an excess of testosterone such as from being proud cut. Also I think horses that are insulin resistant will have cresty necks. If he was overweight at one point in his life and had a thick neck, that hardly every goes away even if he lost a bunch. I've gotten donkeys here before that have had a collapsed neck. They get so fat that their crest actually falls over. Even if they are emaciated, their neck is still messed up.
Spent a whole hour today laying in a pasture, waiting for a sparkling vampire to show up. Alas, I woke up and looked over, only to find a mound of horse crap. Sigh.