I did my friends senior pics and we didnt even bother with grooming her geldings lol. Their legs were muddy and it added a bit of uniqueness to the pics. But, I like the natural look best. Mare and tail brushed out, not braided. Just get him all shiney. :) And put him in a nice leather halter or bridle.
I had my senior pictures done a couple years ago with my horses. They were both clean with pulled manes, and clipped. I just wore a nice shirt and jeans, and used my nice leather halter and the bridle I show in. They turned out great!
A couple of my horses were recently used by a young friend for her senior pictures. I think the look of the horse really depends on whether you are english or western and how 'fancy' you want the pictures to be. When I groomed my boys up, I just gave them a nice bath, made sure the bridle path was clipped, got the mane/tail brushed out nice, and used just a touch of baby oil on their head/neck/mane/tail to give them that little touch of !shiny!
I had found an old bridle and breast collar that were covered with silver that I cleaned up for her to use and both horses looked rather dashing. The only one I have a pic of is Dobe though.
I have a big montage of photos with my old horse that were done for my senior session. We took some with me all dolled up in my show attire, and my horse was wearing the show saddle, show headstall and breastcollar. There are some where I'm on him in a t-shirt and jeans cantering up the hill, and some where I'm on the ground and he's perked up looking at me (because I had treats hidden in my hand). They made some of them into a big montage and they had it hanging up at the mall as a promo for them, so I bought it at a discounted rate.
Oooo! I had a friend who got her senior pics done with her horse also. She wore her prom dress and was sprawled out on her horse bareback. It made for a really neat photo.
We just did senior pics with our horses and as long as you brush and get the big stuff off you are fine. Most of it doesn't show up on the pics and if it does a good photographer can fix it for you easy!