The Swooping Evil is a blue-and-green winged magical creature. Its appearance looks like a cross between a reptile and an extremely large butterfly. When it is not flying with its spiked wings, the Swooping Evil shrinks into a green spiny cocoon.
It can be quite dangerous, as it can feed on people's brains, but also demonstrated being able to deflect at least some spells by flying into their path. It also secretes venom that, when properly diluted, can be used to erase bad memories.
Newt Scamander brought one to New York in his suitcase in 1926, using it to escape from MACUSA with Tina Goldstein and shield them from Aurors that attacked them. It also was used to wrap restraints around Gellert Grindelwald, while he was in the form of Percival Graves.