Douglas Harper's Etymology Dictionary
terato-
before vowels terat-, word-forming element meaning "marvel, monster," from comb. form of Greek teras (genitive teratos) "marvel, sign, wonder, monster," from PIE *kewr-es-, from root *kwer- "to make, form" (cognates: Sanskrit krta- "make, do, perform," Lithuanian keras "charm," Old Church Slavonic čaru "charm").
Wiktionary
terato-
pre. Pertaining to birth defects and similar abnormalities