The oxford English Dictionary tells us that the word "monster" originally meant "a mythical creature which is part animal and part human, or combines elements of two or more animal forms, and is frequently of great size and ferocious appearance."
It turns out that the pumpkins popping up everywhere Halloween is celebrated fit the purpose better than we knew. Thanks to completion of genome sequencing in pumpkins, science has discovered the "monstrous" truth in the history of the bright orange cucurbit:
"between 3-20 million years ago, two different ancestral species combined their genomes to create an allotetraploid - a new species with four (tetra-) copies of each chromosome."
This means the pumpkin originated as a "monster," albeit non-mythical, that combined elements of two different forms. Instead of the usual process, by which each parent contributes one of its two chromosomes to maintain the normal "diploid" genetic instruction manual in the offspring, the ancestral parents of the pumpkin both merged fully into a new type of vegetable. Over the decades, the evolutionary process cleaned up this mess, tossing aside the extra genes until the evolutionarily preferred diploid format appeared and remains in modern pumpkins.
But the traces of the monstrous past remain. Pumpkins have 20 pairs of chromosomes now, well in excess of other cucurbits such as watermelon (11 pairs) and cucumbers (7 pairs). Also, in many plants with a tetraploid in their genetic history, such as cotton or maize, one of the inherited genomes dominates over the other, and will be retained while the second is mostly lost. This was not the case with the pumpkin.
According to the team at Boyce Thompson Institute, "The ancient Cucurbita allotetraploid lost its duplicated genes randomly from both of the contributing diploids. Furthermore, the ancestral chromosome remained largely intact, leaving the modern pumpkin with two subgenomes representing the ancient species that contributed to the paleotetraploid."
In non-scientific speak, that means the pumpkin of today is still a "monster" combining its two long-lost ancestral forms in a single creature. That could certainly explain why the pumpkin, which is "frequently of great size and ferocious appearance," makes such a fitting symbol for the spookiest of days.