Forge version: 14.22.0.2479
Twilight Forest version: 2.4.0.63

Steps to reproduce:

1. Spawn a Hydra
2. Input /kill @e[type=twilightforest:Hydra]

What I expected to happen:
The Hydra (and any other existing Hydra) will be instantly killed
What happened instead:
Nothing happens. The game executes the command, but the Hydra remains. However, if kill @e[type=!Player] is input, the command successfully executes, but the game says it killed the Hydra and all of the heads, even those that do not exist yet, individually.

Doesn't work for some others including the Lich too

The Lich seems to be immune to /kill commands unless he is shieldless

Interestingly, "/kill @e[type=twilightforest:hydra_head,c=1]" will kill off the entire Hydra even when only one head is selected.