I don't know what to call these. They seem to be antonyms, but the words they would be antonyms of don't exist, or mean the same thing, or are used much less often. Perhaps pseudo-antonymns? Untonyms?
flammable / inflammable
incontinent / continent - you never hear about someone who is continent, but they make adult diapers for those who are incontinent.
inclement / clement - you hear about inclement weather, but not clement weather (but the word does exist, I think, and you do hear about clemency)
incentive / excentive - (I heard some people at work talking about "incenting" people)
inundated / undated / dated - "inundated" means flooded. Does "undated" mean "not flooded"?
outcast / incast - someone can be an outcast, but what about someone who is not an outcast – is he an incast?
incest / outcest -
intense / extense - although there is intensive/extensive and intent/extent (how did that happen?)
discombobulate / combobulate - how can you be discombobulated if you were never combobulated?
undaunted / daunted - I've never heard anyone carry on daunted. (Need to check if daunted is a word, or just rarely used.)
feckless / feck - no such word as "feck"
reckless / wreck - reckless sounds like it should mean "not wreck", but it means "wreckful". The word "reck" means "to care for", but I've never heard it used.
nonchalant / chalant -
inept / ept - the opposite of "inept" is actually "apt"
fraud / defraud - to defraud is to commit fraud, but to deice or deplane is to remove something from the base word
impeccable / peccable - I've never heard of anyone having "peccable" taste.
impeach / peach
imbibe / bibe
indescribable / describable - People rarely mention something as being "describable". And doesn't calling something "indescribable" give it a description?
nondescript / descript
inscrutible / scrutible
impune / pune, impunity / punity
indefatigueable / defatigueable / fatigueable
unwieldy / wieldy - "wieldy" is a real word, but I've never heard it used.
insomnia / somnia - there is a band named "Somnia", and apparently "somnial" is a word.
inevitable / evitable - I've never heard the word "evitable" used, although it is in SOME dictionaries; Isaac Asimov apparently wrote a book called "The Evitable Conflict"; when looking up the word, I found that "evite" is a related word, meaning "to avoid" – odd how an eVite
impregnable / pregnable - impregnable means "unable to be penetrated", or "capable of being made pregnant", which seem like opposites to me; some dictionaries do contain the word "pregnable"
impeccable
ineffable
inert
inexorable
unfettered / fettered - fettered is actually a word, but rarely used, and usually used to mean literally "shackled".
incorrigible / corrigible - corrigible is actually a word, but not in all dictionaries