megatherium (n.)

before vowels meg-, word-forming element often meaning "large, great," but in physics a precise measurement to denote the unit taken a million times (megaton, megawatt, etc.), from Greek megas "great, large, vast, big, high, tall; mighty, important" (fem. megale), from PIE root *meg- "great." Mega began to be used alone as an adjective by 1982.

High-speed computer stores 2.5 megabits [headline in "Electronics" magazine, Oct. 1, 1957]

often thero-, word-forming element meaning "beast," from Greek thēr "wild beast, beast of prey," from PIE root *ghwer- "wild beast." Also therio-, from Greek thērion "wild animal, hunted animal."