Tree Mask

The costume comes out of Presents, which enemies drop through the Christmas season. Like the other seasonal windows, that one follows your system date and cannot be triggered from inside the world.

In Hardmode a Present can hatch a Present Mimic instead of handing over loot, and it hits hard enough to end a careless unboxing session. Open the stack somewhere you have room to fight, not in a cramped storage room surrounded by chests.

The mask, shirt and trunks go in the social slots and leave your armor untouched. The costume looks identical on male and female characters, unlike most of the sets. Standing still in it makes a decent multiplayer prank, since at a glance you read as background scenery.