Angry Nimbus

The Angry Nimbus is a rare Hardmode enemy that spawns when the weather is rainy on forest biomes. It will follow players around the map, attempting to kill them with a rain type attack similar to that of the Crimson Rod and the Nimbus Rod. It floats above player making it almost impossible to hit with close-range weapons. They are immune to the Poisoned and Venom debuffs. The Nimbus Rod is a 1 in 15 drop.

Trivia
This could be a reference to Super Mario Bros's Lakitu.