Myth goes back to the Greek word mythos, which originally meant 'word, speech, message' but in the 5th century BCE (Before Common Era) started to acquire the meaning 'entertaining, if not necessarily trustworthy, tale'. 

According to the Oxford English Dictionary (OED), the earliest known use of the noun "mythos" in English was in the mid-1700s. The specific year cited is 1753, in the writings of Samuel Shuckford, a Church of England clergyman and author.  Essentially, mythos relies on storytelling and tradition.  According to the Oxford English Dictionary (OED), the noun "myth" first appeared in the English language in the 1930s.  Specifically, the earliest evidence cited by the OED is from 1830, in the Westminster Review. While related terms like "mythology" and "mythic" are older, the word "myth" itself as a noun is relatively recent in English usage. 

Examples of "mythos" used in a sentence:  
"The Arthurian mythos is a rich source of inspiration for literature and art." 
"Many science fiction stories draw upon the mythos of space exploration and alien encounters."
"The movie attempts to expand the Star Wars mythos by exploring new characters and storylines."
"I love comic books that are rich in lore and steeped in mythos." 
"The tale has come to embody the giant-killing mythos of this tournament more than any other."
"The game's developers meticulously crafted a detailed mythos for the game world, including its history, geography, and mythology."