This is a really common thing in horse photos: the dreaded Lens Distortion. This is Honey, the Christmas Pony I'm working on today. I love her expression in the reference photo at top, but I could tell the camera had made her nose too long! I have to correct that, and here's how I do it.
I measure other things, often less precisely; I will also have to account for her muzzle being too wide, for example, and pare that down as I create the sketch. The side view won't help a lot with that, but I'm experienced enough to figure it out.
I already did a post of simple tips for better amateur dog photos, and all of those apply to horses, but if I can find a horse-photography tutorial I'll link it here.