Looking much like a cross between a lobster and a jet engine, these critters appeared in Harry Potter and the Goblet of Fire. Though they're not all that aggressive per say (to humans at least - they are more than happy to kill each other off) - they don't even have visible mouths - they are dangerous, mainly due to the fact that they are capable of "blasting off", spewing out scorching exhaust in the process. The sight of a poor Care and Feeding of Magical Creatures student holding onto a skrewt's leash for dear life does make for a funny picture however - especially if the student in question is Draco Malfoy.