I was messing around with my Diamond of soulstealing and decided on getting an ancient wyvern. I was doing a task and didn't know how it worked so I asked around. I didn't realize Ancient Wyvern didn't work with the Diamond.
So I would like to suggest an extended list for NPCs that are able to have the Diamond on them, starting with the Wyverns.
All Wyverns
Kurasks
Only ones that I can think of that don't work with the Diamond as these are the only ones I've tried. I'll update this list as I find others that don't work.
I believe this will give the 99 Slayer achievement more use as there's very few slayer tasks with NPCs that people would enjoy having a pet for, I.E. the Ancient Wyvern, Spitting Wyvern, Kurask, etc.