Well, that gave me an excuse to jump into our super-cool internal database, Multiverse (and to send emails to a bunch of people in R&D), to see what I could find out. Here is a developer note for the card:
9/8/12: No Angels here. Could be an Archon, though, or a Demon that serves Athreos.
Okay! So there you go, the Rider became an Archon really early in development. To refresh your memory (or impart you with Magic lore if you didn't know!), an Archon in Magic "is a mysterious figure riding a winged animal."
Wait... what did I just quote? The art description, of course! (You knew that was coming, didn't you?)
Color: White and black creature
Location: A fantastical arch at the edge of the Underworld
Action: In Magic, an "archon" is a mysterious figure riding a winged animal. Start with the reference. Design a masked archon who can travel to and from the realm of the dead. Show a massive, fantastical archway inspired by Greek architecture. Its entrance is blocked with a "veil" of shadows. Show the archon and his winged mount emerging dramatically from these shadows. His winged mount is yours to design, but it shouldn't be a Pegasus. Maybe it's like an undead winged hound that's the size of an minivan.
Focus: The archon
Mood: He transcends the lines between life and death.
Now, the art description uses the male pronoun by default, but Chris Rahn decided to paint the Archon as a female, which you can see in the card's art and is referenced in the flavor text. Oh wait, it's hard to see that she's a she at that scale, right? Well then, here is the full art: