If you're a mounted mage you should always be traveling with a charged spell (corrupt). ALWAYS. Does an MA travel with a no arrows equiped? Does a footfighter run into a duel without a weapon in their hands? Saying that its griefing to have your mount killed because you weren't prepared to defend yourself in an open world with full loot pvp is just complaining about being inconvenienced for your own mistake. I never travel on my mage without having corrupt charged at all times. (yes, that means constantly recharging) It's part of your character build as a mage learn to play the build and I promise you'll do fine.
p.s. If you're complaining about getting griefed while streaming then try streaming with a delay until you're comfortable enough in pvp to fight back. I don't see why one should feel entitled to stream and have mechanics catered to their ability to stream rather than to adjust their streaming habits to the game.
Range of Corrupt is to short and you can only cast while going in a straight line and this is only assuming that your a MM. Your horse would be dead before you even get that chance to get that spell off or you will instead be spending your casting on heals to keep it from being killed. Most of the MA have some casting abilities to heal there mounts so I would assume they would have Purify too.
As for the stream snipping just a comment from watching streamers (I don't stream) we have some players that just go in and snipe the horses of streamers and run off without a fight. Though I have had the same happen to me in the past from fully geared playrs blowing buy and shooting my horse out from under me and not fighting either when I was just out gathering. Just some folks are griefers and get the joy out of making others folks day bad.
As some one mention one of the problems is that horses most the time aren't tuff enough cause of low level and low health when you first get them and most don't get time to level up if your getting them constantly killed our from under you.
For me I think one of the biggest problems is that DEX should account for more in archery, it should be the part of the aiming control while shooting while STR is the DAMAGE. There also needs to be a better way to knock some one off a horse if they have low skills or a low level of horse. This way they should be investing in more than just STR/CON and such.
The other problem is until the QUE issue gets fixed a lot of players can't run grouped up so a lot of them are out solo and easy target for MA's. The best defense would be to have some MA's of your own in the party to be able to chase after them and fend them off those that can't. As a solo player I know I'm an easy target so I keep my head on a swivel all the time when I'm out and about. And no one is saying we don't want PvP, just that it's way to easy for some one to become a MA and fight from horse back against others that didn't gear directly for bow play. Also for folks saying just wear metal armor, not all builds allow this and well any newer player won't have all that gear.
How much does armor actually help on the horse as this could be a better solution of letting them live a bit longer instead of being snipped out from under some one so easly.