When Ethiopia was ruled by an emperor, the aristocracy had a custom of going by additional names separate from their given names. The name for this class of names is (or was, since they aren't extant anymore) horse names (Ye Ferres Sim in Amharic). These names were often given by others to reflect the recipient's personality, especially in relationship to war. Other times, the bearer of the name bestows it upon himself; like in an incident reported by a Scottish traveler when a warrior (in the manner of warriors) was boasting about himself, his lineage and his accomplishments. The name of his horse was prominent in the litany he was reciting. I would imagine minstrels would also either be instrumental in bestowing the names or in popularizing them.
The custom is an old one, with evidence of its use from the 14th century. It appears to have started in the Oromo region of the empire, with some of the founders of the Oromo monarchies only remembered by their horse names. In some cases, horse names serve as regnal names for emperors upon their accession to the throne. This does not mean the horse names used by the Ethiopian emperors were the same as throne names, which are an even more ancient tradition. Emperors started going by horse names during times when the empire was weakened, especially by rebellion. One explanation for the usage of horse names by emperors is that horse names are in a command form rather than a declaratory one; thus, given their military association, are rather aggressive, thereby signaling intent to take command of the empire including its fractious bits.
While the names have a military origin, Mahtama-Sellase Walda-Masqal, the most quoted writer on the subject, divides the names according to their most dominant characteristics. He groups them into those lauding:
- administration and justice,
- kindness and generosity,
- integrity, religion, or cunning,
- anger, forcefulness, bravery, and;
- the color of the horse
The horse names were most often 2 (but sometimes 3) words with the first always being Abba (which meant father or owner of) followed by the name of the horse. Examples include: Abba Koster meaning Father of Sharpshooter; Abba Gersa meaning Father of Recurring Impediment (i.e. impeding the enemy) and Abba Garre Bar meaning Father of Well Mannered.
In relation to names based on the horse's color. The Ethiopians preferred single color horses because in battle, a uniquely colored horse could serve as a target. Only the emperor or the commander usually rode a multicolored horse so that the men can easily recognize and follow him.
Horse names, along with other customs, fell into disuse after the abolishment of the monarchy. Monarchical customs seem to me have a certain grandeur and romance. The older the monarchy, the grander and more romantic it seems. Given the age and continuity of the Ethiopian monarchy, the abolishment of such a grand custom seems like a loss. Republics, especially modern ones, just do not have the romance of the times of swords and horses and castles and chivalry.
References:
Pankhurst, Richard, 1989. The Early History of Ethiopian Horse Names.
Mehari, Krista, 2007. Ethiopia: Sacred Traditions and Visual Culture.
The most referenced work regarding this custom is Mahtama-Sellase Walda-Masqal's "A Study of the Ethiopian Culture of Horse-Names", published in 1969. I did not have access to it for this piece but the 2 noted above quoted extensively from it.