Popular English use of probability synthesizes two distinct concepts, probability proper (what is statistically most likely to happen) and credibility (what is most reasonable or believable) The Aristotelian account of eikos also assumes the two to share overlapping domains. Aristotle deals with eikos at greatest length in his Rhetoric, because it is an important element of persuasion.
In Poetics, Aristotle emphasizes that actions must be probable for the character and situation. He famously stated that "poetry is more philosophical than history" because poetry deals with universals and history with particulars. Specifically, poetry deals with things that are probable or likely but not actual, while history deals with actualities that are often improbable (the more unlikely an event, the more it is covered by historians or journalists -- thousands of people every day doing the ordinary act of working at the World Trade Center was not news; the singluar event of people flying 2 planes into it was news/history).