In case you just want to show it on the iPhone, then upload it to your YouTube (GMail) account, and stream it in the YouTube app on iPhone. (OS X, Windows, Android, iOS).
If you want to put it physically on the iPhone, then use the iTunes program to do so (OS X and Windows).
Or put it in Foto library (Mac, iPad, iPod), and then it automatically appears in the Foto app on all of your Apple devices, including iPhone.
You may have to use a video format conversion tool, depending on the original video format.
A quick and dirty method is to record it with the iPhone camera and microphone when played on another device.