Nymph

The Nymph is a rare enemy that spawns in the caverns. It has the highest HP of any land-dwelling non-boss pre-hardmode enemy.

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

Trivia
The Nymph might be a reference to Left 4 Deads The Witch as fooling the player and attacking whenever they come close, while also the Nymph's color platelet also resembles The Witch.