Kingreaper
Adventurer
However, since a few of you don't like the idea of restricting player targeting, I think I'll use the rule that the rider has superior cover against attacks while mounted. The mount it 10 times bigger than the villain, so naturally it should be able to take most of the attack.
At 10 times larger I'd give the rider total cover, except on rounds when he has attacked (when he'd have superior cover): unless the players figure out some way to get above the mount.
If the PCs are above the mount, let them target the villain IMO. It's not going to be easy to get above/on top of the mount, but doing so would be a very interesting way to complete the encounter.