All Hallow's Eve – Legends rare. All Hallow's Eve was originally printed as a sorcery, errata-ed to an enchantment, and then turned back into a sorcery that used the technology from the suspend mechanic. However, instead of suspend's time counters, All Hallow's Eve goes for flavor with scream counters.
Dragon Mask – Visions uncommon. The art for Dragon Mask was originally intended for Crimson Hellkite, but it just wasn't dynamic enough. So the Mirage team created the idea of a mask that looks like a dragon so the art would have a home.
Haunting Echoes – Odyssey rare. In Odyssey playtesting, there was a time when haunting Echoes didn't contain the words "other than basic lands". Playtesting with the same set's Traumatize soon fixed that.
Headless Horseman – Legends common. Here's something cool: everyone that posts their Magic Online account name in the "Halloween Tricks for Treats" thread (NOT the blog comments section) by 2 pm PDT/2100 UTC on October 28th will be awarded one Headless Horseman promo card. This Legends card has not been released on Magic Online, so here is your first chance to collect him!
Torch Slinger – Zendikar common. By an interesting "coincidence", the two mana that it costs to kick Torch Slinger and deal 2 damage to a creature is very similar to the two mana it would cost to play and equip an actual Blazing Torch.
Umara Raptor – Zendikar common. One of the challenges Mark Rosewater set for competitors in the Pro Tour–Austin edition of "Question Mark" was to list a Zendikar card beginning with each letter of the alphabet (except for X and Y, which don't have any). One of the trickier letters was U, which has only Unstable Footing and Umara Raptor.
Sphinx of Jwar Isle – Zendikar rare. On the Pro Tour, Sphinx of Jwar Isle is popular because it has shroud, protecting it from Terminate and Maelstrom Pulse. But it's popular on the coverage team for three little words:At. Any. Time. If you had one, you could be looking at the top card of your library right now.
Steamcore Weird – Guidpact common. There are only three Weirds in Magic, and they're all in Guildpact. But if they were more common, they wouldn't be all that weird, would they?
Musician – Ice Age rare. Even after Musician leaves play, the costs associated with the music counters have to be paid. Based on Obsidian Fireheart, we can assume that if Musician were printed today, it would say "The music continues to play".
Geyser Glider – Zendikar uncommon. The terrain of Zendikar is constantly undergoing upheavals and changes. The Geyser Glider has adapted to that environment, so when an unexpected lava geyser shoots it into the air, it casually spreads its wings and uses the height to improve its hunting area.
Ob Nixilis, the Fallen – Zendikar mythic rare. Ever wonder what Ob Nixilis was before he became The Fallen? The flavor text gives a hint, mentioning that he's lost his spark. Was Ob Nixilis once a planeswalker?
Shoal Serpent – Zendikar common. The Shoal Serpent's head looks a little odd, doesn't it? That's because at the moment we see it in the art, it's upside down! The eyes are onthe bottom, and the tongue is on the top!
Eternity Vessel – Zendikar mythic rare. Here's a "great" combo: First, cast Eternity Vessel when you're at a reasonable amount of life, like 15 or so. Then slam Lich on the table. Now every time you play a land, you'll draw however many cards your life was at! At fifteen cards per land, winning is left as an exercise for the student.
Sea Gate Loremaster – Zendikar rare. The Lighthouse at Sea Gate is the center of learning for explorers in Zendikar, and it sits in Tazeem, the homeland of the merfolk. The loremaster knows things that even the library does not contain.
Guul Draz Vampire – Zendikar common. In Guul Draz, vampires aren't the monsters; they're the citizens. The vampire homeland is a humid region of teeming lagoons and tangled, fetid swamps, but that isn't always enough to deter daring treasure-seekers from the dangerous land.