Which reminds me ... many War Elephants had Ballistas mounted atop them.
Having War Armadillos with Ballistas mounted atop them would be cool. Equal would be Griffon riders with Rope-hooks and hooked nets.
Warg riders would also work with net-like weapons.
Anyways, other than "normal mounts" ... big n large can mount light seige atop their backs, and any fast mounts can use Rope Hooks and Nets for capture purposes.
In particular, a rope-hook for a Griffon rider could mean hooking onto a soldier, flying high, and dropping them for an insta-kill.