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No, Ensemble Dark Horse is a supporting or minor character that becomes suprisingly popular. This is when you know an unintroduced or recently introduced character will become important because the author has released outside material of them.
A similar idea is an event you know is going to happen because it was hinted at in outside material. Another example from HINABN: everyone knew Conrad was going to become a vampire because he was drawn as a vamp in gallery art, so nobody got upset when he "died".
Late-Arrival Spoiler mentions cases of seemingly minor characters having toys released or being featured on the cover.
Ignorance killed the cat. Curiosity was framed.
Interface Spoiler, the non-videogame version?
Space for rent.
Awesome Fox 888? Awesome FAUX? (Please? Anybody? SAVE ME FROM MYSELF.)
It's sort of like "Narrowed It Down To The Guy I Recognize" - or at least, that's how I remember it. It's when a seemingly minor character in a webcomic is heavily hyped in supporting materials (ex. pinups, gallery art, character sheets, about pages), so you know they'll become more important later on. Hanna Is Not A Boys Name is notorious for this, with Tessa Stone introducing characters like Abner on her Deviant Art long before they even appear.