Like the title says, I think it would be cool if Horses got spooked if you try to tame them while you're morphed in to a hostile mob. Basically, it would buck you off after a second or two, guaranteed, then run away.

Hm, that's seems like a cool idea. Do you have any ideas how to implement this? :)

I think it can be achieved by registering an AI task, something like EntityAIFleeSun or EntityAIPanic on EntityJoinWorld event targeting horses. However, I'm not sure if it's going 100% flawless to work. To be honest, I didn't worked with Minecraft's AI yet.

I agree, registering an AI task is what should be done. I don't know specifically how to do it yet, as I haven't ever tried modifying vanilla mobs' AI.

I'll add this in 1.2 update along with new survival-able features suggested by EnderFlux and @Yashimata.