I know as posted, amulet of nec clickie stops a horse dead if you click the illusion.. Guessing other insta click illusions would do the same.
As others have mentioned /dismount does it as well. But non-illusion insta-clicks will not stop you on a dime. The reason why AoN & /dismount stops you is because the model changes.
Tried it out. Both snail and carpet from TBL appear to stop instantly. I.e. if I start casting immediately upon releasing forward, there is no interrupt. The rhinos from EoK (the gray one in premium and the gold one for group content progression) have quite a bit of momentum. Interestingly enough, my SK's lvl 85 AA horse also seems to stop on a dime.
The TBL carpet you get from doing all the quests definitely does not stop instantly while in combat. It's what I use on my warrior and I never use it in combat for that reason.
I agree...I hate this garbage. If I am in a full run and want to stop and cast it takes a good 3-4 seconds easy. As far as I can tell my mount isn't a '77 Cadillac, so get rid of this bs.