Danica and Zorica, the Zorya, daughters of the Dazbog, an old Slavic god and anthropomorphized versions of the evening and morning stars.
Because of their heritage, they are strong and clever and used to easy success, something that doesn't come quite as easily to them at Monster High. They can act a little bit "above it all"--they are stars, after all!--but they aren't above some petty rivalry with their fellow students. Their best friends are each other, but being around others like them is helping to draw them out a bit.
Also, the pet is mythologically accurate--according to lore, the Zorya guard the massive winged hound, Simargl (probably stolen from the Byzantine Simurgh, a winged, gryphon-like creature) who legend has it will devour the cosmos if he is ever freed. I thought it'd be funny to make him one of those scrawny, whippy, yappy-type dogs. X) Cute lil' destroyer of worlds, yes he is.
(And because I get asked this every time, NO they are not meant to look like Luna and Celestia from MLP. >:[ Love the show, but the fact that one is white and one is purple does not an MLP reference make. Also, to my shame, I have not read Neil Gaiman's American Gods, so I have no reference point for how his Zorya might differ from mine. I heard he made up a third Zorya for the "midnight sky" which has no mythological basis, but he's Neil Gaiman, so I imagine he can do what he wants.)