The Fire Archer Eidolons are labelled 'Medium' in the main window, but "Heavy" (and not Eidolon) in the summary pane.
Honestly I wouldn't normally share this here, but I'm reporting the bug to the devs on the official forums, and their forums aren't letting me upload the screenshot. So I figured I'd share it here and embed it using this site's URL instead, and share it with the good people of !rts while I'm at it.