I don't know a whole lot about Hatsune Miku, but apparently these are all characters from that world in pony form. They haven't relased any other info about them yet, but I'll get something up if they do.
(Update: Characters are Hatsune Miku, KAITO, MEIKO, Megurine Luka, and the twins Kagamine Rin & Len. according to submitter Karbol)
