The snowman on the surface of the snow is a lure. The real Snowman tunnels underneath the snow and dirt, creating a labyrinthine network of tunnels underneath parts of Snowdin. Its body has become a semi-solid mass of ice shards, diamond hard and capable of destroying virtually any material, if given enough time.
Anyone who approches its lure will fall through the snow and into the monster below’s waiting maw. Its prey is quickly sanded down and incorporated into its own mass, absorbed into the now blood red shards of ice.
Beware any Snowmen in your path. You will not live to regret sentimentality.