This photo is from Wikipedia and is an excellent example of a horse with a crested neck. This is not an extreme case of a crested neck but you can definitely see the extra muscle and development of the neck from the withers to just before the poll.
The crested neck is common in stallions, ponies and draft breeds. You may also see it in Morgans. Some mares may also have a crested neck. The cause is usually excess fat deposits above the nuchal ligament in the neck and the horse may be obese.
Excessively crested necks may even fall to one side. So you have a horse with this large amount of fat that kind of hangs to one side with the mane. This is an extreme case and is not very common. Some of this excess fat can be removed through a proper conditioning program, but this may or may not make a difference.
In conformation, the ideal neck is about 1/3 of the horse’s body length when measured from the poll to the withers, for the correct length of the neck.
