Nightmarity

Nightmarity, Generosity in Death, is a unique Unicorn that can found in Whimsyshire and Whimsydale in Diablo III.

In combat, it has Arcane Enchanted affix and shoots volleys of multiple arcane missiles that spray in front of itself.

Trivia
The name of this monster is a possible reference to Nightmare Rarity, one of the characters in My Little Pony.