If you present a poster and submit an abstract and the school has all the abstracts available to view online, would you include the abstract as a publication in ERAS? Would it fall under a "Non-Peer-Reviewed Online Publication"?
No, regardless of whether your school decides to post the abstract online it’s still a poster. You know it’s a poster, I know it’s a poster, and a residency program would know it’s a poster. And it’s not clear to me that a non peer reviewed online publication is actually better than a poster anyways—anyone can write whatever they want for a nothing website and call it a “publication,” but actually putting together a poster and presenting it in an academic setting and answering questions means you did something and at least is a more accepted kind of currency on CVs.
Regardless of the chronology, this is all pretty clearly the same work product from the same project. You should put it in one place or the other, not both. Based on what you have described, my recommendation is that you put it under poster.