pipersack

It's gonna be Coco Pommel, brother. Bank on it.

There is a possibility that King Sombra is not a true unicorn at all, but an evil presence that takes the form of a pony to inspire fear in his victims by reflecting their worst attributes in their own image. When he appears as shadow in the show, he is formless. Maybe he is in fact a shadow spirit or evil spirit with no native form. His horn and face structure are noticeably different from other unicorns in the show. I think either he was born as a true unicorn but underwent a kind of magical metamorphosis in his quest for more power that granted him this power at the cost of his former unicorn-status, or he never was a unicorn at all, but an evil shadow that assumed a unicorn-like form. In this way, he has no cutie mark, because only the pure ponies have them (changelings do not, as they are pony-like beings but not ponies, nor do windigos, which also appear pony-like in form but are in fact evil spirits in pony-like forms).

Additionally you can view G1 evidence of comparison by viewing G1 drawings here http://ts2.mm.bing.net/th?id=HN.607996399391868379&pid=1.7 and figurines here http://www.amazon.com/Vintage-My-Little-Pony-SUNDANCE/dp/B00F3R06AY makes Megan appear about 20% taller than the G1 ponies. This would make a human pre-teen girl about the same height as Tavi's instrument she is usually shown with. If we assume Megan to be 10-12 years old, or about 11, the average height for an American girl of that age is 58 1/2 inches. This would make Megan just under five feet tall, and the G1 ponies about 46-50" by comparison, again confirming the four foot average pony size. I know its not G4 canon, but the size comparison of the candy cane seems to be in very close agreement with this analysis using G1 information as well.