Here is the 'fault':
(I had a remarkable amount of trouble finding pictures that accurately represent what I'm talking about)
Compared to a 'normal' gut:
I don't know if you could actually call it a fault, or if it's just a sign of a horse that's race fit? Like having a flat abdomen? I don't see it in every TB that's race fit, but I do see it primarily in TBs that just came off the track. What's it called? Thanks!