The English language is full of words with uncommon properties.
Unpaired words have no opposite equivalent. They have a prefix or suffix that suggests you could form an antonym by removing the prefix or suffix, but forming their opposites will take more work than that. You can be “disheveled, but not “sheveled.”
Unpaired words occur because certain words fall out of common usage (“ruthless” and “ruthful”) or because one word from a presumed pair is borrowed from another language (“dismayed” comes from the Old French “desmaier”).