9-letter words that end in -nym:

• pseudonym – a fictitious name (like a writer’s pen name or nom de plume).
• cryptonym – a code name or code word.
• heteronym – One of two or more words that have identical spellings (hence the ‘hetero’ prefix) but different meanings and pronunciations, such row (rō), a series of objects arranged in a line, and row, a fight, pronounced rou (like cow or now).

8-letter words that end in -nym:

• ethnonym – name of people or ethnic group.
• retronym – A word or phrase created because an existing term that was once used alone now needs to be distinguished from a term referring to a new development (so I assume retronym itself is a new word), such acoustic guitar in contrast to electric guitar or analog watch (or just plain ‘watch’) in contrast to digital watch.
• tautonym – A taxonomic designation, such Gorilla gorilla, in which the genus and species names are the same; commonly used in zoology but no longer in botany.

7-letter words that end in nym:

• acronym – I hope you already know this one.
• synonym – And this one, too.
• homonym – This one is tricky as I, too, often confuse this one with homophones (the suffix shows the difference, if you pay attention). A word the same as another in sound and spelling but different in meaning, such as light, meaning either illumination or of little weight. (The English language is full of these!)

Not to be confused with:

• metonym – A word that denotes one thing but refers to a related thing: “Washington is a metonym for the U.S. government” and “plastic is a metonym for credit card.” 
• antonym – You better know this one!
• toponym – A place name or a place of a region.
• paronym – A word that shares the same derivation as another word.
• hyponym – A word whose meaning is included in the meaning of another more general word. For example bus is a hyponym for vehicle. 
• allonym – A name, often one of historical significance or that of another person, assumed by a person, especially a writer (maybe I should call myself Jack London, haha).
• caconym – An erroneous name, especially in taxonomic classification (oh, yeah, I’ll bet this is popular among wordy nerds…)
• autonym – A name by which a people or social group refers to itself. Also a piece of literature (in journalism and publishing) published under the real name of the author.

6-letter words that end in nym:

• eponym – A word or name derived from a proper noun: atlas, bowdlerize, denim, Turing machine (modern day computer) or one whose name is or is thought to be the source of the name of something: Alexander Gardener is the eponym of the gardenia.
• anonym – An anonymousperson or apseudonym.
• exonym – A  name by which one people or social group refers to another and by which the group so named does not refer to itself, as in the K in K-pop (Korean music).

Whew! Not an exhaustive list but exhausting to write it out! Language, like life itself, is in constant motion; humans continually add new words (like retronym) and discard old or bad words (that’s too long a list for here but maybe another post) to suit our communication needs. What’s in a name? Why, -nym of course!