I bought an eventer to play in the hunter-seat equitation ring. 
When he came, he was fit, fit, fit. But we did develop his topline differently with the result that he is certainly “crestier”- stronger-muscled down the top of his neck- now than he was 10 years ago. In terms of fat deposits along the crestline, metabolic changes in his teens did help with that. Through the rest of his body, changing from endurance work to shorter durations with higher intensity develops the muscles differently.
He is an Irish Sport Horse with considerable bone and the widest-sprung ribcage I’ve ever seen for a horse of his stature. It’s easy to look at him and say “that horse is fat!” but he’s a 5 on the BCS.
For those of you more familiar with the import market than I am, I wonder if some of the “crestiness” on the high quality high performance geldings might have come from being gelded later, if at a young age the horse looked like it may be worth keeping entire.
