Both 'onomatopoeic' and 'onomatopoetic' exist in English and are well formed, from different Greek abstract nouns. The root poi- "make" could take either the suffix -í�? (which appears in English as -y or -ia) or the suffix -ēsis. The words onomatopoií�? and onomatopoíēsis are attested respectively from Hellenistic and Byzantine times. The corresponding adjective endings are -ic and -etic.
Now it gets confusing. It seems in -poeia, -poeic the -e- is part of the root poi-, regularly rendered in Latin as poe-. But in -poesis, -poetic the Greek uses an alternative stem of just po-, and here the -e- is part of the suffixes -esis, -etic. Okay, now even I'm confused. *squints and memorizes*