Werewolf

The Werewolf is a hardmode enemy that spawns only during a full moon. In this time they become as common as Zombies, so be aware that you may get swarmed.

When a Werewolf attacks, it has a chance to give the Bleeding debuff. At close range, it will attack the player by leaping or lunging at them, closing distances quickly to catch unwary players off-guard. Werewolves can also open doors.

You can transform into one by wearing the Moon Charm.

Trivia
Through common lore, the main weakness of werewolves is silver, but using this strategy in Terraria is not recommended because silver weapons deal little damage at all to werewolves, so you're better off using pre-hard mode endgame items.