Pop up on Mac from AVG
After further investigations i found that LoaderAgent is part of the AVG LinkScanner installation. So download the installer and use the uninstaller in the dmg. It removes the LoaderAgent.app and supplemental files and folders.

Just for record keeping, my earlier answer:


  Which AVG Antivirus did you install? Did you get the installer package from a reliable source?
  
  The latest AVG Antivirus for Mac doesn't create a folder named AVG in /Library/Application Support/ - at least for me using the free installer and not subscribing to any additional services or installing AVG Cleaner or PrivacyFix. Instead the folder is named com.avg.Antivirus. Especially it doesn't contain a LoaderAgent.app in any of the subfolders. Moreover I can't uninstall it "using the AVG website".
  
  To get rid of the remnants of your AVG deinstallation, download and install "LaunchControl"
  
  Open the app and search for avg in Global Agents/Global Daemons.
  
  
  
  Uncheck all radio buttons to disable the jobs
  
  
  
  Quit LaunchControl, go to /Library/LaunchAgents and /Library/LaunchAgents, check for plists containing avg and open Terminal.app
  
  Enter sudo launchctl unload &lt;-Drop the *.plist here and hit Enter to unload all related services. If the plist is a symbolic link or an alias right-click it and choose "Show Original" and use the original plist. You have to enter an admin password then.
  Repeat that for every plist containing *.avg.
  
  Then enter sudo rm &lt;-Drop the *.plist here and hit Enter
  Repeat that for every plist containing *.avg.
  
  Navigate to /Library/Application Support/ with
  cd /Library/Application\ Support and hit Enter.
  
  Then enter sudo rm -dfR AVG and hit Enter.
  
  Restart your Mac.