Listening for entity removal?

Hello wonderful helpful inhabitants of the development forum.

I develop a plugin called BetterPassives. It modifies the passive mob spawning system to make it, well, better. However, I'm running into an issue with it. The plugin splits passive mobs into two types: wild and tamed. Tamed ones are supposed to never dissapear, while wild ones despawn and spawn like they did in beta 1.7. My problem is that the tamed ones are vanishing.

I have repeatedly tested it, and I believe that my plugin's entity removal system is not the issue. For some reason, some other something is removing them. So I would like to listen for their removal and cancel it. However, I cannot figure out how one would go about this. the DamageCause enum has no DamageCause.REMOVAL value, and there's no EntityRemoveEvent, right?... If anyone could point me in the right direction I would really appreciate it.

Thank you!