Deer Thing

It only fires in snow or ice, and unlike most summons there is no time of day restriction, so you can call the fight in broad daylight. Build the arena in the snow before you craft the item, because walking into a boss in a biome you have not flattened is how most first attempts end.

Frozen chests down in the ice caves carry them at a generous rate, which makes a sweep of the ice biome the lazy route to the fight. If you are exploring down there for the other chest loot anyway, you will likely have one in your inventory before you ever bother gathering the ingredients.

Dropping one into Shimmer breaks it back into parts, and the evil ore you get follows whichever evil your world runs. Drunk worlds alternate day by day, so if you want a specific ore out of it, decraft on the correct day. The name follows the Things from Don't Starve, which is also where the boss came from.