Hellhound

Only during the Pumpkin Moon, which you start yourself with a medallion at night, and only once the event reaches its third wave. There is no other way to meet one and nothing else in the world spawns them.

It runs and leaps the way a unicorn does, and its top speed is close enough to a player in good movement boots that a flat chase is a losing race. From the third wave onward several arrive at once, so the mistake is agreeing to fight them on the ground where they are fastest.

They cannot drop down through platforms. A raised platform floor, even a single row above the ground, removes them from the fight entirely while you shoot down at them. Anything with reach below you turns the wave into free progress toward the next one. Their banner needs double the usual kill count, which sounds harsh until you see how many turn up across a well run night.