When you have to have a crupper AND a breast collar to try to keep your saddle from sliding all over the place you may have a mutton withered horse. When your trainer gets on your horse and says, "Holy moly, this horse doesn't have withers, he's got shoulders like a frickin' bull!", you can be pretty sure you have a mutton withered horse.
She's not this fat anymore but she's a good example of mutton withers.
This is the guy the trainer made the bull shoulder comment about. If I forgot his breast collar it was guaranteed that I would end up in the dirt before the day was out because his saddle would just ROLL.