The french word manteau means coat,porte meaning to carry, so porte manteau means a clothes carrier...literally a leather trunk. There are many french words which have an opposite meaning in English and are called “faux amis,” or “False Friends.” One must be very very careful with some of them in order not to become a red-faced idiot in genteel company.
The word does make sense though, in that a saddle bag is just two separate bags that have been linked together, as portmanteaus are generally two words that have been linked together.