Forge version: 1.12.2-14.23.0.2512
Twilight Forest version: MC 1.12.2 - 3.3.202

Steps to reproduce:
Just look for Mo' Creatures' animals and monsters, you'll see them

What I expected to happen:
I expected that only Twilight mobs spawn in the Twilight dimension

What happened instead:
Mo' Creatures' mobs spawn too, and Mo' Creatures' monsters spawn at any light level, as seen in these
screenshots:

I pasted the same screenshot twice, here's the second one:

is that not the point? Mo'Creatures adds its spawns to all dimensions. there isn't really much we can do here.

Closing as this is not within our control. Check the mods that are spawning these mods for a blacklist, such as restricting dimensions they are allowed in, as that is the best way to prevent this.
If a blacklisting option or spawn control option does not exist, report to those mods.