How to get the horse back

Here is a link with the xbox one manual. I know it looks like a fishy link so if you want to download it yourself you can go to (http://help.ea.com/en/dragon-age/dragon-age-inquisition/?category=game-play-in-game) and at the bottom left is a link for all the manuals.

http://d2ro3qwxdn69cl.cloudfront.net/manuals/dragon-age-inquisition-manuals_Microsoft%20Xbox%20One_en.pdf

Try to hang back a bit while your tank (or other characters) start attacking, so they grab the enemy's attention (aggro). Your warrior tank should have abilities like Challenge and War Cry - that'll definitely get their attention off you.

I'm playing a mage myself. When attacking a rift, I switch to my warrior or any of the other characters and simply use them to launch the first few attacks. Then switch back, and you shouldn't have aggro.

Like someone noted in the comments - it does seem that enemies who are currently not attacking you, but have not been aggroed by anyone (e.g. not attacked, or taunted), will shift their focus to you once you start disrupting. The only way to solve this is by focusing it down, or by taunting it with your tank.

Also, I've noticed that, when using a different party member, the option to Disrupt Rift also appears - but I haven't actually tried that, so I don't know what happens.