猛然 (měngrán) and 猛地 (měng de) both mean 'suddenly/abruptly': 他猛然回头 = 'He suddenly turned around.' 猛烈 (měngliè) means 'violent/intense' for storms, attacks, or criticism.
memory
犭(dog/animal radical) + 孟 — a fierce, aggressive animal. The animal radical appears in many characters for fierce creatures.