Stallions who are mature can get BIG jowls and thicker, beefier necks. But not all will or do. I had a gelding who was gelded late and had huge jowls, the thickest damn neck, he was VERY stud-y in appearance.

Your friend sounds like they haven't met many mature studs. :P

So sometimes a "stud" will look different from a mare but not always?

Yep. It sometimes goes by breed too. I've never seen an Arabian stallion who was huge-joweled or had a massive thick neck, and I grew up with a close family friend who ran a Polish Arab stud barn so I saw a LOT of mature Polish Arab studs.

I see it most often in some Thoroughbreds and halter-bred stock horses.

Some Mustangs get stud-necks too.

If you're looking for tertiary characteristics like in humans, you're going to be sorely disappointed.

at a distance there really arent too many... being around horses you pretty much have to 'look down' at their abdomen. stallions (on racing breeds its more pronounced than work breeds) tend to have a bit of an early up-slope while the females have more of a belly. mares that have foaled in the past never gain back their 'tight tummy'. if you are really studying them though, mares have a slightly wider pelvic structure but stallions hide that structuring with muscle mass.

mid distance you see slightly sharper features on the face (jaw points) on stallions are slight more pronounced but not a lot.

other than that, equines are pretty generically shaped to help hide who's who in the herd

Thank you, I felt there had to be some difference and was fustrating being told they are exactly the same.

There are actually some very minor frame differences you can see as well, but with specific individuals it can be hard to visually assess without a direct comparison. Mares generally have a more horizontal rectangle shape from the side (shorter legs compared to back length), whereas stallions typically are more squared/balanced (back to leg length generally close to equal). Geldings that are cut young are the opposite of mares, (shorter back with longer legs.)

Again, it varies quite a bit from breed to breed, and even among individuals, but this is what I've observed over years of seeing them in person. The differences are very minute, but they're there!

The most noticeable differences are what's mentioned above, with stallions often having more of a crest and stronger jaws, and mares being a little more streamlined. (again, that can vary, though, with a number of factors)

Thank you for the add to my little research project :)

You're welcome! :D I remember stumbling across several very useful and informative sites while doing some research of my own a while back. Will link them if I can dig them up!