TL;DR: Frankenstein's monster and zombies are both slave metaphors

Generally speaking, the Halloween costume canon distinguishes Frankenstein’s monster from zombies. However, I suggest that the zombie myth and Mary Shelley’s literary monster, entering the Western imagination at roughly the same time, have an intertwining history: zombies influenced Mary Shelley’s depiction of her creature (and our own interpretations of it), just as Frankenstein’s monster has significantly contributed to our modern concept of the zombie.

The word zombie was coined by West Africans to describe the cadaver-like victims of Voodoo priests, who would poison their targets and force them to work in the fields. This myth travelled via slave ships to the West Indian island of Haiti and took root; the enslaved Haitians emphasised the power dynamic already inherent in zombies, and this folklore came to express their own feelings of a “social death” in enslavement. Finally, zombie folklore arrived in England in the late 18th century in Bryan Edward’s History of the British West Indies (1793). We know that Shelley owned this book and read it at least twice in the years leading up to her writing Frankenstein. Of course, we can only speculate whether the zombie myth directly influenced her monster, but I would argue with certainty that it has maintained an indirect influence ever since.

Frankenstein’s monster and zombies both begin as an expression of slavery. The zombie myth is born as a kind of slave narrative, and many critics argue that Shelley writes Frankenstein from an abolitionist perspective. Shelley adopts certain tropes of slave literature: using orientalist language to racialise the creature, establishing a master-slave relationship between Frankenstein and his monster, using acquisition of literacy as a metaphor for emancipation, and fire as a symbol of revolt (which was a major tactic used by slaves in the Haitian revolution). The middle section of the novel is written from the creature’s perspective, giving him a voice and humanising him for the reader, who gradually overcomes their initial prejudice and sympathises with him. Replace ‘creature’ with ‘slave’ in this preposition, and Frankenstein is clearly a text about slavery, written to inspire abolition in an English audience.

Zombies and Frankenstein’s monster continue to intertwine throughout their adaptation and evolution. In every reimagining, zombies and Frankenstein’s monster become more and more similar. As I have established, they begin, on a basic level, as reanimated corpses with a background of slavery. In the 1930s, when horror films became zeitgeist, zombies and Frankenstein’s monster were interpreted in much the same way, in Victor Halperin’s White Zombie (1932) and James Whale’s Frankenstein (1931). The posters for each film are almost identical, emphasising the conjoined nature of both concepts at this point. In both films, the creatures are no longer presented as victims, but as villainous and threatening monsters. Significantly, they are also now presented as speechless; in the case of Frankenstein’s creature, reduced from persuasive prose to bestial grunts, emphasising his monstrosity and removing all pathos from the audience. I would argue that Frankenstein’s monster is a zombie primarily because modern pop culture treats him as one; we cannot interpret them separately because they have such a shared evolution, with the same roots in slavery and Western fear of the Other.

Well yeah. How is he not?

Well he's technically a horde of zombies, rather than just one zombie.