Slow Down videos already recorded

Not sure if you are asking about playback or actually rerecording at slower speed. If you want only to playback at a lower speed then SpeedupTV does this well.

https://itunes.apple.com/en/app/speeduptv/id386986953?mt=8

There are a lot of similar apps in the app store. Most 3rd party video players allow you to do this as well. For instance nPlayer has an option to play back at slower speed.

There are also apps (for instance VideoShop) that allow you to edit and rerecord at a lower speed. Search the app store for 'slow motion video' and you'll see a number of possibilities.