Nymph

The Nymph is a rare monster that spawns underground. When first encountered, it will appear as the Lost Girl and pretend to be a helpless NPC that needs to be saved. When you get close, however, it will transform and attack as a Nymph. It's a dangerous enemy as it does high amounts of damage and easily surprises new players. After the first encounter it won't appear as a Lost Girl again. It is arguably the rarest creature in Terraria and is extremely hard to find.

Tips

 * It cannot attack from a distance, nor can it jump very high. As such, using a ranged weapon, preferably from somewhere it cannot reach you, is an easy way to defeat it.
 * It is easier to fight with a Harpoon or flail, as Nymphs can be stun-locked.
 * It appears that it spawns more frequently in the console version.

Trivia

 * The Lost Girl appears to be completely naked, which is a unique trait amongst all humanoid NPCs.
 * In Greek and Latin mythology, a nymph is a lesser female deity with the power to animate nature.
 * Although the skin and eye colors change, the hair remains relatively the same.
 * It could be a reference to the witch in Left 4 Dead. Just like in Terraria, it appears as a helpless girl, but when you startle her, she will attack you with high damage.
 * As the Nymph is one of the rarest enemies in the game, its Monster Banner is one of the hardest items to obtain.
 * She shares a striking resemblance to the great fairy in Legend of Zelda Twilight princess.