Note that these are what's most common in this world's canon, but not every unicorn, pegasus, crystal, or earth pony will look exactly as has been described. Images above provided by mod Anon to show what an average individual across races may look like.
Earth ponies generally can be either large or small, and all have a strong build with large chests, rounded bellies and ears, and bigger more muscular hindquarters. Their legs tend to be thicker since traditionally they are constantly working on the ground, which is where their source of magic comes from.
Speaking of magic, while earth ponies aren’t the strongest with their magical abilities, they have an affinity with magic in the very earth they walk on. They can make crops grow faster and stronger, and without this, all the ponies would most likely starve. They were also the ones to develop holding things with their hooves, which other species have a more difficult time with. They have also been known in the past to make potions and medicines.
Pegasi tend to be smaller, though this isn’t always the case, than the other races. They have lithe bodies and thinner hooves, though strong back legs and thicker hindquarters due to kicking clouds and controlling the weather conditions. They also can be fluffier with more feathers, but this is a spectrum, as with wing span.
Most of their magic tends to be concentrated into their wings and hooves. It’s due to this magic that they can maneuver clouds about. They also have the ability to control wind speeds with their wings, as well as pick things up and hold things with their wings.
There are also a nocturnal kind of pegasi, the bat pony, which are very similar to their feathered kin except that they have leathery wings, slit pupils for seeing in the dark, and often tufts of fur in place of extra feathers. Longer canine teeth are also common with bat ponies, though length varies from individual to individual.
Obviously, unicorns have horns, though these can vary depending on pedigree, just like Pegasi wings have multiple options, so too do unicorn horns. Some are long and sharp, others are short and stout? They could be curled, swirly, spiky, etc. Unicorns tend to have larger chests, but are thinner everywhere else, and their ears are longer than those of Pegasi or Earth ponies, as well as pointed. Traditional unicorns tend to have long tails with hair tufts at the end that correspond with their manes, but Unicorns with mix-heritage can have traditional pony tails.
Unicorns are arguably the strongest magic users out of the ponies. All the great majority of their magic is concentrated in their horns, and even in their tails as they can hold things in them. With their powerful concentration of magic, they create spells and such to make the lives of ponies easier. Their horns are protected with a membrane beneath the skin and fur that helps the magic flow as needed and protect it from outside forces. If a horn is damaged, it tends to cause a lot of problems, and spells either don’t work, or backfire spectacularly. It is fortunate to note that broken horns can heal, but if broken off completely, it is highly unlikely to grow back.
These sparkling ponies share a common ancestry with the Earth Ponies. The Northern Mountain Ponies which lived in the Northern snowy Peaks near where the Crystal Ponies would create their empire.
The similar features shared by Earth Ponies are made all the more glamourous with shimmering pelts and eyes that seem to resemble jewels and gems. This quality varies amongst every member of the kingdom, but one may have a harder time seeing it amongst those with brighter pelt colors as it contrasts with darker hues much more easily.
