Correct spelling, explanation: considering the origin of the word, we come to the conclusion that creator is correct. The form is derived from Latin as creator, not creater. It means that suffix or is added to the verb create in order to make the noun, referring to a person that does something. In Middle English, the form existed as creatour but since the 13th century, the word has been used as creator. Thus, the correct spelling is creator with a suffix or, not er.

Definition of creator:
noun, a person that brings something new into existence,
The women were the creators of the newest clothes in this fashion show.
Dickens was the creator of many wonderful characters in his novels.

Incorrect spelling, explanation: making nouns from verbs, we add the suffix er or or to the base. But there is no rule saying when it should be used er ending and when or ending. However, the origin of the word indicates in what way it is spelled correctly. The form has been known since the 13th century as creator. Nevertheless, it has its roots in Latin and it has survived in this unchanged form as creator. Thus, creater is simply a misspelling probably because of its pronunciation as /kriˈeɪtə(r)/ when the sound similar to vowel e is heard. To sum up, creater is spelled incorrectly, whereas the errorless form is creator.