Ivermectin has research going back to the mid-2000's showing it has anti-viral properties.

EDIT: Thought I'd come across a study indicating that from 2006, but I can't seem to find it. Let's settle on mid 2010's instead because the point is mostly that there was research showing anti-viral properties before the pandemic started, and therefore it was a very logical thing for frontline doctors to try on their patients.