After upgrading to El Capitan, OS X imported my email accounts from my iPad I just upgraded from Snow Leopard to El Capitan on an iMac from 2009. I guess my iPad was connected or something, but I found that the operating system's mail application now had the accounts that I had on my iPad! This is a huge no, no. I wasn't even asked for it to do this, not to mention I don't know how it could have done this since only iTunes, AFAIK, syncs with iOS devices. How do I disable this?
More than likely, your Mac synced with your iPad and got data from iCloud. Go to Settings -> iCloud, and see if Mail is turned on. If so, and you do not want these to sync together, turn it off. Then, you can simply delete the accounts from the Mail app on your Mac. 

In regard to your question about other "surprises," while you're in Settings on your iPad, take a look around any other areas, such as Notes, Calendar, etc. that you may not want to sync to your Mac.