First up, I upped the contrast on your pic so more detail comes through.
I have drawn some lines onto your pony ref' I understand that she is not the type you are going for, but she does have very good definition for structure ref' The lines are where I think you are missing on Todd.
The second lot of pink lines are the muscles, and jaw pouches. This is the part that will change from horse to horse, in shape and size, and wether the horse has a mouth full of grass.
The next two blue lines are mapping out the nostril cavity, on your boy he is resting, so the definition should only be slight, which I think you have acheived, if you want him snorty, they would of course expand as he drew in scent and air ready to take action.
You can see i have brought the top blue line all the way up to her eye, this is a groove made by the skull and nasal bones on a horses head. You can see on the following image where the soft part would be, and how it merges into the bony parts. The orange is the little sausage that forms the inside of the nostril, I cannot see from your pic' wether you have it there or not. :-)
Once again, my mad keen PS skills! :-)
Thank you, Karen! It certainly did make sense and I have yet again more updated photos of Todd: