Nymph

The Nymph is a VERY rare enemy that spawns underground. When first encountered, it appears as the Lost Girl, who seems to be a helpless NPC in need of rescue. When a player gets close, it transforms and attacks as the Nymph, a dangerous enemy that deals high damage (for a pre-Hardmode enemy). Once the Lost Girl transforms for the first time, subsequent spawns will skip the Lost Girl form, and appear instead as hostile Nymphs immediately. 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. Use a ranged weapon from an out-of-reach position.
 * A Harpoon or Flail can be effective, as Nymphs can be stun-locked. Even with its high health, the Harpoon can kill a Nymph in moments with no risk to the wielder, due to its extremely high rate of fire, so long as it is used at short range.
 * Any high rate of fire weapon will be very effective, as they cannot approach you enough for their attacks to hit.

Trivia

 * In Greek and Latin mythology, a nymph is a lesser female deity with the power to animate nature.
 * The left half of the Nymph Banner is of the Lost Girl, while the right half is the Nymph monster.
 * The Nymph Banner, being a rare drop from an incredibly rare enemy, is still one of the rarest items to obtain in Terraria, even after the 1.3.0.2 update which saw banner drop frequency increased fourfold.