@Frustration in Excelsis  
Eeyup! If that’s not a scary thought.

So as it turns out, this might not even be the first case of a “background alicorn” in MLP.
 

 
This is from “Escape from Katrina”, or so I’m told.

The most logical choice for the next princess.

Looks like Wind Whistler became an alicorn by accident too XD

“Only someone with no feelings could remain calm at a time like this!” declared Gusty loudly. Others of the Flockfell clan and their hangers-on murmured in agreement, for who would naysay the great hero of Tambelon?
 
Yet softly and cold as the morning dew spoke Wind Whistler the lore-giver. “I meant that we need a plan of action”. Her words came forth as tempering ice water upon a new-forged blade. And as she spoke all the assembled ponies gasped, for a wisp of the ancient starry magic of her proud lineage had alighted upon her brow for the space of one wonderous heartbeat. A new kind of resolve began to form in the minds of the amazed gathering.
 
Some who had witnessed Wind Whistler speak at that time, later said that for an instant the sparkling outline of the mythical Alicorn kind had alighted upon her in a moment of blessing.