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. [source] The Nymph may also be a reference to Left 4 Dead's Witch.