A word for words that are often seen in their negative forms

These words are called Absent Antonyms or Unpaired Words.

absent antonym: ...negative-looking words commonly found in the English language whose presumed positive forms are rarely, or never, found. [2wheels.org.uk]

unpaired word: ...is one that, according to the usual rules of the language, would appear to have a related word but does not. Such words usually have a prefix or suffix that would imply that there is an antonym, with the prefix or suffix being absent or opposite. [wikipedia]

Some of them end up with matching antonyms as a result of back-formation (sometimes as a jocular back-formation like gruntled as opposed to disgruntled), or had antonyms in the past which fell out of use.

There is also the term lonely negatives used at mentalfloss.com. However, they are defined as "negative words whose positive partners have vanished or never existed in the first place." Common examples: Disgusted, disgruntled and disheveled.