The carpus consists of several small bones arranged in two rows and 3 associated joints between the forearm (radius and ulna) and the cannon (metacarpus). Due to its resemblance to a human kneejoint, it is often refered to as the knee, however anatomically speaking it is a different joint entirely (equivalent to the wrist).

An extinct ancestor of modern horses, Dinohippus emerged ~12 mya from late Merychippus species. It was a one-toed grazer and its skull, teeth and limbs were already very similar to our modern horse. It appears that the passive stay apparatus, which allows horses to stand without muscle power, first appeared in Dinohippus.
During the late Pliocene, Dinohippus was the most widely distributed horse in North America and it is assumed to be the direct ancestor of the genus Equus.

The oldest known ancestor of the horse. It lived in the eocene era between 60 and 45 million years ago.

Hyohippus evolved out of Miohippus and was a specialised, pony-sized browser. Its feet ended in tree toes, which was an adaptation to soft forest floor. It had a long neck and comparatively short legs. It lived around 17 to 11 mya in what is Nebraska, Colorado, and Montana today. Together with Megahippus and Achniterium, it belonged to the group of Anchiteria, which thrived over millions of years but eventually became extinct.

This genus appeared around 20 mya in North America. With a shoulder-height of around 1m it was of a similar size and stature as Parahippus. Merchyhippus was similar to Parahippus and it is difficult to distinguish the fossils from each other as some species of both genera lived alongside each other.
Its molars were high-crowned and covered in hard cement and pronounced ridges and folds formed structured surfaces, suitable for a diet with a higher proportion of grasses. It Is assumed that Merchyhippus lived so some extent in savannah habitats. Like Parahippus, Merychippus carried its weight on the tip of the middle toe; the two lateral toes were reduced but still present. The footpad became reduced and the springy system of supporting ligaments more pronounced. Radius and Ulna fused, so leg rotation became impossible.
Merychippus gave rise to ~20 different species, comprising specialised grazers, but also mixed feeders and browser, which continued to thrive in a variety of habitats.

This Equid emerged about 40 mya during the colder, more seasonal climate in North America. It the had a shoulder height of about 50cm and only had three toes on each foot, with the middle toe being more pronounced. Compared to its ancestors, its back was less arched, its legs and neck were longer, and its skull was slimmer.
Its teeth indicate that its diet consisted mostly of leaves and some grasses, less of soft buds and berries like the diet of its ancestors. Its premolars were very similar to its molars, indicating that it was better adapted to chewing coarse plant material.

Members of this genus where slightly larger than Mesohippus (around 50 kg) and had a longer skull and limbs. They lived in what is now North America, alongside Mesohippus before the latter became extinct.
Its lateral toes were reduced even further and they probably only touched the ground during fast running or when moving on soft ground. Miohippus was the first Equid to have the typical ridges on their molars which we can see in modern horses and which facilitated grinding coarse plant based diets.
Up until now, only one or two Equid genera were present at a time. Miohippus was the origin for the first larger diversification of horse species and it gave rise to at least three genera with many different species, some living in the forests, some on the open plains.

About 50 million years ago at the beginning of the middle Eocene, Orohippus appeared. It was still of similar size and stature as Hyracotherium. Front limbs still ended in four, hind limbs in three toes. Over time, Orohippus' teeth showed some adaptation to a slightly coarser diet, including leaves and maybe some grasses. One of the premolars became more similar to a molar, providing more grinding surface.
During this time, the temperatures slowly started to decrease, which marked the onset of the transition in vegetation.

Like Miohippus, this species had three toes on each foot. However, Parahippus was the first Equid to carry its weight exclusively on the last bone of the central toe (the pedal bone). The claws gradually developed into hooves and the previously cushioning pads were incorporated into the hoof capsule, forming the frog and the heel bulbs. Some of the bones in the distallimb fused and thus provided more stability; this reduced the range for rotational movement of the limbs.
The muscles on the limb moved more closer to the trunk and long, strong tendons developed which reached into the distallimb, stabilised the latter and enabled an efficient transmission of muscle power to the limbs.
Parahippus is considered a mixed feeder, somewhere between browser and grazer. It had high-crowned molars and was probably one of the first horses that included a larger proportion of grass in its diet and lived to some extent on open plains. It is also assumed that Parahippus was one of the first horses where the teeth erupted throughout its life.
With a shoulder height of about 1m, it was the largest Equid to have evolved so far.

An extinct genus of the Equidae family which lived in North and South America, Europe and Asia in the Miocene period approximately 7 million years ago. It was the first of the modern one-toed ungulates with one hoof and only vestigial remains of the other toes. It was larger than its ancestors with a shoulder height of almost 120 cm. It lived on plains and steppes or in light woodland.

Exudate formed as a result of tissue degeneration and inflammatory response to an infection with pathological micro-organisms (bacteria).

Knock-kneed / carpusvalgus: a conformational fault of the forelimbs when viewed from the front. An imaginary line dropped from the shoulder passes through the lateral portion of the carpal joint; the carpal deviates medially.

Hyracotherium, also known as Eohippus or the dawn horse, was a genus of forest-dwelling herbivores about the size of a fox. Its distribution ranged from America to Eurasia. Its front limbs ended in four toes, its hindlimbs in three toes. Its feet were padded, similar to a dog's paw and the claws already showed similarity to hooves.
The low crowns of Hyracotherium's teeth and the lack of a structured grinding surface indicate that this animal was a browser, mainly feeding on soft leaves, buds, and berries. Analysis of the stomach content of a fossilised individual found in the Messel Pit in Germany supports this theory.

It lived in the eocene era between 60 and 45 million years ago.