It was a M&B patch that introduced it if I remember correctly.
Cannot remember the exact length. But I know that the staffs don't rear any more if that is any help.
It needs to do pierce damage on the stab to rear.
Was gonna post the old requirements needed for a horse rear, but wiki no longer exists (so much for trying to make it useful) and I couldn't find anything from a search so...
From what I remembered it was something like:
Has to be a polearm
Has to hit head/neck
Has to be 141 or longer (this is what changed)
Has to do pierce damage.
Horse has to not be doing an animation with an animation value higher than X (aka, a dying animation will not turn into a rear animation for obvious reasons)
Horse has to be moving.
Weapon hitting has to be from a dismounted person.
I think that was most of it.
