Pixie

The Pixie is an enemy that was introduced in the 1.1 update. It has 150 health, seems to spawn in The Hallow and drops Pixie Dust. The Pixie comes only in a yellow color- The other colors seen are summoned from the Fairy bell. It also appears to be capable of only hovering, not flight, enabling it to glide up walls and hover over traps, but not move freely through the air.

Trivia
The Pixie's design is based on fairies from the Zelda series.

The Pixie was originally thought to be an upgraded orb of light due to it's relatively friendly appearance and high amount of light emission.

This later turned out to be true as the Pixie is an enemy, and the Fairy bell summons a colored pixie that serves as a light source.