Works in 1.13, 1.13.1 and 1.13.2
With this datapack you'll have the posability to loot, with a Looting 3, all the mob's heads, including Illusioner, Zombie Horse and Killer Rabit.
You have also an Advancements section with all the mob heads collection.
You can summon a Illusioner by simply renamed a Vindicator "illusioner" ("i" not "I" for illusioner)
You can summon a Zombie Horse by simply renamed a Skeleton Horse "zombie" ("z" not "Z" for zombie)
You can summon The Killer Bunny by simply renamed a white rabbit "killer" ("k" not "K" for killer)
if you want to see the drop percentage check out my website here