Horses have always been the means of transport. They can run very fast and it is that in previous times they had wings and could fly. Even in Greek mythology we have Pegasus, the flying horse. Their main Characteristies is their loyalty, industriousness and swiftness. A good example of these qualities is Kantaka, the horse of Siddhartha Gautama. When the future Buddha lift his palace in order to become an ascetic, his horse realized that he will never see his mas6ter again and died of a broken heart. He was reborn in one of the heavens.
In Buddhism the horse is symbolic of energy and effort in the practice of Dharma. It also symbolizes the air or prana which runs through the channels of the Body and is the vehicle of the Mind.
In that sense we can quote a passage from one of Milarepa's "Song of the galloping horse of a yogi."
"In the mountain hermitage which is my Body,
In temple of my breast
At the summit of the triangle of my Heart,
The horse which is my Mind flies like the wind"
"He gallops on the plains of great bliss.
If he persists, he will attain the rank of a victorious Buddha.
Going backward, he cuts the root of Samsara.
Going forward he reaches the high land of Buddhahood.
Astride such a horse, one attains the highest illumination"
[translated by Losang P. Lhalungpa]
The so called "Wind-Horse" symbolic of the Mind the Mind its vehicle the wind, can be ridden upon. That means that we have the possibility of controlling the Mind and wind and guide it towards any direction and at any speed that we wish.
The neigh of a horse is also symbolic of the Power of The Buddha to awaken the sleepy Mind for the practice of Dharma.
There are some stories of the Bodhisattva Lokesvara taking the shape of a horse in order to help Sentient beings.
In buddhist iconography the horse support the throne of the Tathagatha Ratnasambhava and carries the chariot of Surya, the sun God. The horse is also the vehicle of many other deities and Dharma Protectors, such as Mahali, and there are horse-faced deities, such as Hayagriva.