It is said to be a kind of Onibi (fox fire).
鬼火の一種とされる。 - Wikipedia日英京都関連文書対訳コーパス
Then, it is said that a lot of foxes appeared and set a foxfire to illuminate the road.
すると無数の狐が現れ、狐火をともして道を照らしたという。 - Wikipedia日英京都関連文書対訳コーパス
More specifically, it is an illustration of tactics using fire in which one ties the legs of a bird on the back of a fox and set fire to the wings and send them to the castle of enemy.
具体的には、火攻めの兵法として、狐の背に烏の脚を縄で括り付け、その翼に火を点けて敵城に放つ戦術の図解である。 - Wikipedia日英京都関連文書対訳コーパス
In "the family record of the Kazanin family," it is written that the shrine was enshrined by FUJIWARA no Mototsune, an adopted child of Yoshifusa, as an guardian god, but in "Doyuki" above, the following story was recorded as a continued story by Moronari; when Shosenko (posthumous name as Daijo Daijin), Mototsune, was in a low position, he saw a fox being beaten by several people so he asked them to give the fox to him and he released it, and the fox appeared in his dream and pledged to remove disasters such as fire if he gave it a place to live, so he gave the fox the present place of enshrinement and made it as a kenzoku (messenger of the gods) of the Munakata deity.
『花山院家記』には良房の猶子藤原基経が守護神として祀ったものと記すが、上掲『土右記』には師成の話の続きとして、「昭宣公(基経)の身分がまだ低かった時分、数人の童に捕まり杖で打たれている狐を見かけたので、それを乞い受けて解放すると、夢中にその狐が現れて、住む場所を賜れば火難などの災害を除く力になると誓ったので、現鎮座地をあてがって宗像神の眷属とした」と載せている。 - Wikipedia日英京都関連文書対訳コーパス
"Burabura" in "Hyakki Tsurezure Bukuro", a collection of ghost prints by Sekien TORIYAMA, is described as the ghost of a firefox by the author, but it is drawn as a lantern, and so there is a view that it is a kind of a lantern ghost.
また鳥山石燕の妖怪画集『百器徒然袋』にある不落不落は、石燕による解説文では狐火の妖怪とされているが、画図では提灯として描かれているために提灯お化けの一種だとする説もある。 - Wikipedia日英京都関連文書対訳コーパス