Scenario: You create an custom illager beast with a hostile mob on top. When you fight this mob and block the illager beasts charge the roar the illager beast does will hurt the mob on top causing the two mobs to become hostile towards each and eventually causing the illager beast to kill the mob on top.
I have tested this with several mobs and every time this is the out come when I put any mob on top of the illager beast.
What I would suggest it to make the riding mob immune to the illager beats roar.
Please watch the video I included. Us map makers would really appreciate this!