Pure white, silent, only vaguely humanoid, with pitch black little eyes and gnarled, wooden arms. Everything about a snowman is inherently eerie, yet our culture seems to regard them as wholesome, harmless creatures by default, and exclusively tied to the winter holidays after Halloween is over.
The innocence of these icy entities is so ingrained in our subconscious, exceptions are almost always played for comedy or some sort of ironic twist, as if anything about an animated mass of snow is any less ominous than a whispy ghost or a haunted pumpkin.
When snow creatures do get taken seriously, they're typically more along the lines of an "ice golem," removed as much as possible from a stack of snowballs with a carrot nose. Should we really count these as the same type of creature? It's hard to know exactly where to draw the line, but I will say that a snowman shouldn't have to look immediately menacing to qualify as "spooky" if we already accept simple sheet ghosts as such.
Granted, I do like that something so bizarre looking is widely regarded as adorable, and it just goes to show the amazing power of context. I'm certainly not going to say that I'd prefer snowmen entirely reframed as "scary" entities, no, but perhaps we could see some more acknowledgment of them as "spirits," and yes, even let them in on Halloween as a "monster," however cute a monster it may be.
What is a snowman, then, if we're to consider it a true monster species and not just an abstract seasonal mascot? Like most Halloween creatures, it's probably a special sort of spirit that inhabits a body of snow once that snow has been "personified," and likely just wants to engage in some innocent, ghostly mischief of startling small animals or children...though if we do want to get darker, there are surely those snowmen and snowwomen malevolent enough to just plain freeze people to death, or maybe even eat them.
The one obvious weakness of a snowbeing is that, if it can't maintain its snow body, it has nowhere to go. This certainly shouldn't be the "death" of the creature, however; it just remains invisible, incorporeal and harmless until cold enough weather comes back, and some individuals may even be bound to some other item, like a hat or a scarf, rather than to the snow itself.
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The longer a snowman remains animate and the colder it gets, the more powerful the spirit becomes, and those that enjoy a more bogeyman-like lifestyle probably become stronger still with every rotten little child they gobble up. Eventually, a snowman should become powerful enough that it doesn't even melt in the heat anymore, constantly surrounded by its own personal bubble of freezing cold. Obviously, even the most powerful individuals still don't like hot, sunny weather and would rather hang around in icy mountains or caves most of the time, but they may come out of hiding as early as the very beginning of fall, and only help to usher in even colder weather.
The whole point of the fall harvest was to prepare for the winter; the harshest, bleakest and deadliest time of the year for many parts of the world. What could have ever been creepier than a piece of winter itself getting up and walking around?