It really is such a shame that source of this spell is secret - the established naming convention is already perfect. Gehenna's Golden Hounds - Alliteration and ego! Ours would be - hold on a second, I have to look up very specific shades of grey.
Marengo? I like it. (#4C5866)
Mathilde's Marengo Horse! Alas, it cannot be. But, hm.
Wikipedia says this-
"Marengo (c. 1793–1831) was the famous
war horse of
Napoleon I of France. Named after the
Battle of Marengo, through which he carried his rider safely"
It's perf-
Urk.
It does still have "Mare" in the name, this is salvageable. I just have to assign the spooky ghost horse a gender. Would "Nightmarengo" be too busy? The word already means "Grey," that's a lot of overlap with "Night". But less so with "Knight," eh? Knightmarengo.
Each section of [Knight/Night/Nightmare/Mare/Marengo] is applicable to the spell. And the parts of the name blend into each other, as do the "Knight" and the "Mare" of the apparition. Boom.
[X] Marengo
[X] Knightmarengo