Mod that creates a follower plane?

See if this is what you want.

https://flyawaysimulation.com/downloads/files/18930/fsx-ai-formation-toys-v4/

It's almost that, the only problem is that the formation on this add-on don't follow me. But thanks anyway o/

Closest that I could find.

there is a mission that gives examples of AI that follow another AI or you can make him follow you, I have tried it and it does work...it uses simvar, and once installed will allow all missions made with simvar work

check this like from Jim Kier(author of simvar)
http://www.jimkeir.co.uk/FSXME/Examples/SimVar%20Expo.html
there is a download on the page

SimVar Extension Details
Availability : Download from FSAddon at http://www.jimkeir.co.uk/FSXME/MissionDisplay.zip

Cost : FREE including commercial use, but not commercial distribution

Support : http://forums.fsaddon.eu/viewforum.php?f=22

Example missions using SimVar : http://www.jimkeir.co.uk/FSXME/SimVar Examples.msi

hope these help you

hope you like this post, has some AI that you can chase and shooot at if you have a weapons pack or i assume tacpack as well
http://flyingspirit.freehostia.com/Airpower/index.html