The Flatwoods Monster is a famous UFO-related cryptid that while not necessarily violent in the sense of wishing deliberate harm on others is noted for being a creature that (supposedly) caused danger to those it encountered.
The Flatwoods Monster was described as over 10ft tall and having a strange ace-of-spades shaped head, colored red, while its body was green - depending on the witness it either had no arms or very small stubs with sharp claws.
The creature was supposedly encountered by several people during the course of September 12th, 1952 and some witnesses claimed the creature hovered towards them emitting a shrill hissing sound, causing them to flee - others claimed animals such as dogs would flee with their tails between their legs in fright.
However the real danger of the Flatwoods Monster did not reveal itself until later when many of the witnesses reportedly became ill and upon going to the doctors were diagnosed with symptoms similar to those of people who had been exposed to mustard gas (whether this was deliberate exposure or not will likely never be known).
Yet on a more rational basis the symptoms the witnesses had are also common to those caused by post-traumatic stress - at any rate it suggested a frightening experience. One witness claimed her daughter was so distressed upon seeing the Flatwoods Monster she had to be admitted to a local Hospital.