Snow Balla

You will never meet one in the wild. Snow Ballas arrive with the Frost Legion, which never starts on its own: somebody has to set it off with a Snow Globe, and those come out of presents opened around Christmas. They only appear once a world has reached Hardmode, so an earlier legion will not include them.

The snowballs it throws do damage and then leave a snow block wherever they land, including midair if the throw went high. Over a full event the ground in front of your house fills in, the gaps you were dodging through close up, and your own shots start clipping terrain that did not exist a minute ago. Fight the legion somewhere you do not mind reshaping.

The same habit is exploitable. Drop one into a pit it cannot hop out of and it keeps throwing at the walls, giving you a renewable supply of Snow Blocks the way a trapped Antlion gives you sand. It is one of the very few things besides you that can place blocks.