So is this a reference to the comic Sunstone? As in Sunstone by Stjepan Sejic? [Note that while this image is "safe," the Sejic comic is NSFW.] This is in the background of a panel (not relevant to the pony comic's story), and Rainbow Dash is talking about a "sun stone" with no further explanation or context to a random background pony whose hair style is like Ally's from Sunstone.

Assuming this is a reference, had anyone else ever caught it before? (The comic is from 2013, but I couldn't find anyone talking about it by Googling.) And would anyone have any guess as to what that cutie mark is? I can't figure that out.