#hart

General:

Longer backs than real deer + more even hips/shoulders for an easier ride

Longer, stronger legs and lower knees than real deer, similar to horses

Somewhat bigger hocks and hooves

More realistic colors and wide, fallow/red deer-like faces

Stag:

Brecilian Sure-foot

"Wild type" coloration with strong markings and a prominent dorsal stripe

Smaller, more compact antlers than the original harts (based on Megaloceros)

Stags are rarely kept intact, mostly due to aggression. While good breeding and training can result in an even-tempered stag, they still remain unpopular as mounts due to their large, uncomfortable shoulder humps, while their antlers block the rider's line of sight. If worked at all, intact stags are better suited for pulling carts or carrying loads

Antlers may be sawed off completely for safety, but usually only the points are cropped

Antlers and velvet are annually harvested, effective for crafting and potions

Castrated Stag

Pride of Arlathan -- smaller than most other breeds, built for endurance

Dilute color similar to mushroom in horses; less stark dorsal stripe, pale stockings rather than dark

Another, more uncommon genetic modifier: spotting. Since certain deer species (and most young) are known for their spots, I thought they suited these guys more than stripes

Dilute colors may cause lighter/amber eyes

White neck/chin caused by a gene similar to splash white/SW1 in horses

Once castrated, a stag's antlers grow in small and asymmetrical, won't shed, and typically stay in velvet year-round. The velvet nor the antlers can be safely harvested in these deer

Valericorn or "cactus" antlers are fairly common

Hind

Red Hart

This breed is known for its bright red fur, which lightens the dorsal stripe and usually the legs (though not in this one)

This hind also has white markings, which are very common in any breed. (Again, replaced the stripes on the rear with something more cervine -- in real deer, piebaldism, or a partial lack of melanin, causes a wide variety of white spots, splotches, etc., including this "tuxedo" pattern + socks. It's especially prevalent in domestic reindeer)

Unlike horses, there is notable sexual dimorphism in harts. Hinds are smaller and more lightweight than stags, but the difference isn't quite as severe as in real deer or wild harts. Hinds are actually preferred as mounts due to their small shoulder humps and lack of antlers