Download or Stream Video

Yes you can! You can use UnityWebRequest or WWW to download the video from web URL in the form of bytes in persistent Data Path. And then use that path as value of "URL" in VideoPlayer component introduced in recent version of Unity (2017.1 I guess).

For Downloading the video:

 IEnumerator testVideoDownload(){
 var www = new WWW ("http://clips.vorwaerts-gmbh.de/VfE_html5.mp4");
 Debug.Log ("Downloading!");
 yield return www;
 File.WriteAllBytes(Application.persistentDataPath+"/videoFile.mp4",www.bytes);
 Debug.Log ("File Saved!");
 }

Setting Up Video Player

Create a Quad. Add VideoPlayer component to it. Choose Source to URL. Set "Play On Awake" to false.

Script for Video Player to Play the video.

 using UnityEngine.Video;       //Make sure to add this
 .
 .
 public GameObject VideoPlayerGO;
 VideoPlayer video Player;
 
 void Awake(){
 videoPlayer=VideoPlayerGO.GetComponent<VideoPlayer>();
 }
 void Start(){
 PlayVideo("videoFile.mp4");
 }
 void PlayVideo(string fileName){
 videoPlayer.source = Application.persistentDataPath + fileName;
 videoPlayer.Play();
 }

I have used following plugins for streaming in iOS :

To Stream audio: https://assetstore.unity.com/packages/tools/audio/audio-stream-in-ios-182585

To Stream Video : https://assetstore.unity.com/packages/tools/integration/video-streaming-in-ios-98645