Pony Muscle Tips?

Okay, so currently I’m in the process of working on a deviantArt commission which is in two parts: the first part is already uploaded (I would provide a link to it for you guys, but dA is experiencing technical difficulties ATM.), but the second part is where I need help.
 
You see, the artist who commissioned me has a muscle fetish, and wanted me to draw their ponysona all big and muscular. However I dunno how to pull it off right without it looking ugly and overall unappealing (Certainly don’t wanna disappoint my customers, now do I?). So to all you muscle artists on this site, do you have any helpful tips on drawing pony muscles?
 
P.S: The image being commissioned is going to be in black-and-white.

you also need to understand how an actual horse is put together and exaggerate it then apply it to the pony model.
 
here is a tumblr to help you get started with that and it has a lot of refference. HORSE STUFF
 
Here’s a drawing I did a while ago using that approach.
 

Please just use horse photos as reference. So many people disregard this.

Thanks for the tips, everyone. I took the advice some of you left me and-after 2 hours of Googling “Muscular Horse Photos”-I’ve finally finished the commission. Tell me what you guys think!