The Lutece twins easter egg in S4E23. I note that I originally had believed their mark to be a cage, but now that I've checked the iTunes version, I'm more convinced its the standard hourglass mark, slightly recolors to stand out on the pale coat color (which still works within the reference). This is the clearest single shot of that mark: imgur.com/dxGTl6V
which convinced me it was the hourglass.
My Little Pony Friendship Is Magic by
, Copyright Hasbro.