Proposed Canon
First Mentioned: IPTSF Text 77
High Horses are centuarlike Mount Units with a horse's head resting upon a human torso resting in turn upon a horse's body. They are capable of changing colours. They were used by Haffaton, Faq and presumably El-Efbaum. They are tunnel-capable Units.
From the picture in IPTSF 80, High Horses come in male and female varieties.
 Real World References
A High Horse is said to be something people get on when they're being almighty at others.
The fact that it can change color is a reference to the 1939 movie adaption of the Wizard of Oz, in which a horse that periodically changes colors draws the carriage that takes Dorothy into the Emerald City.
In IPTSF 80, the female High Horse appears to be nagging the male. A nag means both an old, broken down horse as well as to annoy with persistent faultfinding, criticism and complaints.