Nymph

The Nymph is a rare enemy that spawns in the caverns. It is tied with the Shark for the highest HP of any pre-hardmode enemy.

The Nymph will have the name 'Lost Girl' until approached, then it will turn into the Nymph and start attacking.

Tips

 * Be careful when facing this enemy - it can be very hard to kill. A good weapon to use would be a Ruby Staff.

Trivia
The Nymph is possibly (though unlikely) a reference to the Canadian supernatural series Lost Girl as that's the name of the Nymph until approached. In the series, Nymphs are natural creatures that often attack humans. [source] The Nymph may also be a reference to Left 4 Dead's Witch.