Giant Bat

Giant Bat is a bigger, healthier and stronger version of the Cave Bat. It uses the same bat AI and also spawns in the cavern layer. The Giant Bat can also afflict players with the Confused debuff, causing controls to reverse.

Like many mobs introduced in 1.1, the Giant Bat will only spawn after the Wall of Flesh has been defeated.

Triva
The Giant Bat's appearance and color scheme, along with the fact that it can confuse you, seem to be a reference to the Pokemon, Golbat.

It is one of the most annoying enemies in Terraria