Yes, the dead of Cybertron have risen many times. The reanimated form of Optimus Prime returned in the creepiest G1 episode, "Dark Awakening". Starscream and Wreckers leader Impactor shambled about in the UK comics. While not technically a zombie, the fused form of Ratchet and Megatron from Marvel's Transformers #70 was certainly one of the most disturbing images from that series.
Starscream. Who ever thought that the most back-stabbing and weasely Transformer of all time would have an immortal spark? After his death in "Transformers the Movie", Starscream returned to haunt the world of the living in the G1 episode "Starscream's Ghost" and his immortality would be carried on through to Beast Wars, Energon and Transformers Animated.
Everyone hates spiders, especially giant, maniacal, mechanical spiders. Tarantulas was especially creepy as a force to be reckoned with during the Beast Wars.
Blackarachnia started out as a female copy of Tarantulas, but quickly emerged from his shadow to become one of the most popular Transformers of the modern era. Blackarachnia has always walked the line between good and evil and continues to do so in Transformers Animated.
The Frankenstein-like Autobot X was created from spare parts by Sparkplug Witwicky. When Sparkplug's son, Spike, is critically injured, mad scientist Wheeljack tranfers Spike's mind into the hulking monstrosity, and the fun begins! Autobot X's first and only appearance was in the "classic" G1 episode "Autobot Spike". Autobot X even watched the movie "Frankenstein" during the episode. I guess the writers wanted to make sure the kiddies who were watching understood the analogy.
In the G1 episode "The Dweller in the Depths" we got to meet the Quintessons' early experiments, the Trans-Organics. The most powerful of these creatures was the Dweller, an energy leech who created "energy vampires" out of his victims. There certainly were quite a few horror movie type episodes in that crazy third season.
The character(s) Savage/Noble was a pretty unique addition to Beast Machines series. The werewolf-like Noble, who would transform into the dragon called Savage, was actually Megatron. Megatron's attempt to rid himself of his organic side (from Beast Wars) resulted in this strange creature. After Megatron's spark was freed from this form, the Noble creature still somehow lived, but was later killed while fighting Megatron.
Some of my favorite G1 toys were the Terrorcons...Hun-Gurrr, Rippersnapper, Cutthroat, Sinnertwin and Blot...who could combine into the monstrous Abominus. I think the Terrorcons can be considered dragons, although Blot's creature form defies explanation.
Of course the most famous Transformer dragon would have to be Transmetal 2 Megatron. After merging with the spark of his G1 namesake, and then taking a bath in hot lava, this new and improved Beast Wars Megatron eventually went on to conquer Cybertron.
Who can forget the Pretenders? (Yes, I know some of you want to.) The Decepticon Pretenders did have some pretty good monster shells, though. The first three were Skullgrin, a bone-faced minotaur; Bomb-Burst, a giant vampire bat creature; and Submarauder, an undersea gillman. Although they never appeared in the US cartoons, all three played a major role in the Japanese Masterforce series as Dauros, Blood and Gilmer respectively.
Another Decepticon Pretender of note is Bludgeon, master of Metallikato, a Cybertronian martial art. After a fine showing in the Marvel G1 comic, the skeleton-faced Bludgeon became a fan favorite.
Although my colleague Deinonychus recently covered the best Transformers dinosaurs, I'd be remiss if I didn't mention the big daddy of them all, Trypticon. Able to give Godzilla himself a run for his money, Trypticon can not only crush a city, but transform into one as well.
That's the end! Remember, always trick-or-treat with a buddy, wear reflective clothing at night, and only eat wrapped candy. Oh, and if you're being chased by monsters, never, ever run into that old, abandoned house at the end of the street. That never works.