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 is likely 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.

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