源頼光はかつて、藤原道長の父である兼家が新造した
二条京極第の落成の折、賓客に馬三十頭を贈ったという。
頼光は藤原摂関家と密な関係を築いていたのである。

牛の角を備えた黒馬・京極。
頼光の魔力を拠り所として実体化する乗騎。
その名は、おそらく贈答した三十の馬に関連付けて
頼光が与えたものであろう。

主人と共にカルデアの風紀を守るため、
京極は、次の命令を待つ。

霊基を変えたことでカタチを成した主人の存在が、
たとえ、一夏の幻にすぎずとも───

Minamoto-no-Raikou apparently gifted thirty horses to guests when the new Nijyo-Kyogoku was completed by Fujiwara-no-Kaneie, Fujiwara-no-Michinaga's father.
In so doing, Raikou managed to establish an intimate relationship with the Clan of Fujiwara-Sekkan.

Kyogoku, the black horse adorned with bull horns.
A riding horse created using Raikou's magical energy as its power source.
Raikou likely chose his name in reference to those thirty horses.

To protect Chaldea's decency with his master, Kyogoku awaits her next order.

Even if his master's existence was only possible by her changing her own Spirit Origin...and even if it may be an illusion that lasts only for a single summer...