Send iPhone slow-mo video to non-iPhone user
Share with an iOS/macOS user

To share a slow-motion video with another Apple device, just use the Messages app and ensure you're sending the message as an iMessage (that means blue bubbles, not green ones). If the video sends as a text message, the slow-mo effect might be lost, so ensure both users have iMessage turned on.

Share with an Android/Windows/Chrome OS user

To share a slow-mo video to an Android/Windows user via MMS or another service, you'll need to export it to iMovie like Brad mentioned.  Here's how to do that:


Take your slow-mo video in the camera app.
Open up the iMovie app, which should be preinstalled. Just ask Siri for help.
Go to the Projects tab hit Create Project.
Choose Movie, select your slow-mo video at the top of the photo picker, and hit Create Movie at the bottom. Your slow-mo video will be converted to a standard video (while maintaining the slow-motion effect).
Hit Done in the top-left corner to finish your movie.
Tap the export button (it's the center button of the bottom toolbar) and send your video with the Messages app (or Whatsapp, FB Messenger, whatever). 


Using this method, users of non-Apple devices will see your video actually in slow motion.