A person named Jack Dahlgren claims on Quora he invented the term in 2002 when he worked at Intel.
Here's what he posted at above link:
I believe that I am the one who coined this term back in 2002 when I
was at Intel and we were evaluating Sharepoint Team Services. Based on
the unfortunate tendency of Sharepoint to escape spaces in names with
[underscore] characters (among other things) I recommended a policy of
using underscores to replace all spaces so that URLs would be slightly
shorter and much more readable.
Given the existing "camelCase" name with humps in the middle, I called
it "snake_case" or if there were two flat spots, I jokingly called it
"road_kill_case". Considering the size of Intel and my interactions
with Microsoft product team, it is possible that this is the origin,
but it is such a simple phase that I think it could have been invented
independently elsewhere too.