Nymph

The Nymph is a very rare enemy that spawns in the cavern layer. When 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).

Achievement
Deceiver of Fools is the achievement you earn for killing a Nymph for the first time.

Tips

 * While the Nymph is indeed one of the rarer enemies in Terraria, it spawns more frequently than is initially apparent, and is often missed due to its initial stationary behavior. It often remains in dark areas, without making its presence known by moving to attack players as most enemies would. Once a player acquires a Lifeform Analyzer (and keeps an eye on its information display), the Nymph's actual spawn rate can become more apparent, and its drops easier to acquire.
 * It cannot attack from a distance, nor can it jump very high. Use a ranged weapon from an out-of-reach position.
 * A Harpoon, Flail or Yoyo 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.
 * A Hunter Potion, Lifeform Analyzer or both can be useful for finding Nymphs.
 * In Expert Mode, the Lost Girl form has twice the health of a normal friendly NPC. This makes it suspicious even on a first time encounter.

Trivia

 * In Greek and Latin mythology, a nymph is a lesser female deity who served the god of nature, Pan.
 * The left half of the Nymph Banner is of the Lost Girl, while the right half is the actual Nymph monster.