Rearing is only dangerous if it's not done properly. Canter is too :P
A horse that rears without permission is just like a horse that runs off without permission.
Or as they said on a swedish forum:
"Oh no! Don't teach your horse to JUMP! What if it just jumps without an obstacle? Or starts jumping out of the pasture! :O
But most people/kids teach rearing by pulling the reins and kichk the sides, and at the same time they don't have a good sense of timing when it comes to correct the horse if it does it when it shouldn't... that tends to be bad.
But think of the spanish riding shool and levade.. it's a rearing that comes out of collection. And it's safe since its trained right. Not that the rearing has to come out of collection, but it should have a cue that's very hard to mistake, and it has to be a well trained horse. Not all horses are fit to learn it.
Sometimes tho, I know horses that has been taught to rear at command because they did it by disobedience before.. and it helped. (just like you can teach a barking dog to be quiet by first teaching it a cue for barking)
Enough of rearing :P
Other tricks are bowing (learnt that to one horse), spanish walk, carrying things, following you and backing with you, lie down, sit etc :)