We do not currently know of any antonyms for portmanteau word.
The noun portmanteau word is defined as:
A word which combines the meaning of two words (or, rarely, more than two words), formed by combining the words, usually, but not always, by adjoining the first part of one word and the last part of the other, the adjoining parts often having a common vowel; for example, smog, formed from smoke and fog.
