I have this same thing. What is this? Login items that I have include Dropbox, Better Touch Tool, Fantastical, and this. aznboi91, do you happen to have any of those applications installed? Does anyone else have any of those three installed and not see this "Popup" login item? (I figure this will help with ruling out what it may or may not be.)
Edit: I found out what it is. My university uses Pharos so that you can send print jobs to their printers - Popup appears to be the name of its background helper application. I suppose I'll leave it just for incase I ever need to use their printers again.
For those wondering how I figured this out: unlock preferences (lower left - so that it won't be grayed out anymore and will let you actually select it), then right click the item and choose Show in Finder. Bam - it took me right to Pharos.