A type of species found in a few Zone of The Seven Realms like the Pyrron zones.
They are several sub species of Horned Fiend too.
Horned Fiend are mobs who will always stay at the same spot. When you are 5 levels higher or lower, they will not attack you anymore.
A summary of all the Horned Fiend in the game: Category:Horned Fiend.